re(3erl)
NAME
re - Perl-like regular expressions for Erlang.
DESCRIPTION
This module contains regular expression matching functions for strings and binaries. The regular expression syntax and semantics resemble that of Perl. The matching algorithms of the library are based on the PCRE library, but not all of the PCRE library is interfaced and some parts of the library go beyond what PCRE offers. The sections of the PCRE documentation that are relevant to this module are included here. Note: The Erlang literal syntax for strings uses the "\" (backslash) character as an escape code. You need to escape backslashes in literal strings, both in your code and in the shell, with an extra backslash, that is, "\\".
DATA TYPES
mp() = {re_pattern, term(), term(), term(), term()}
Opaque data type containing a compiled regular expression. mp()
is guaranteed to be a tuple() having the atom re_pattern as its
first element, to allow for matching in guards. The arity of the
tuple or the content of the other fields can change in future
Erlang/OTP releases.
nl_spec() = cr | crlf | lf | anycrlf | any
compile_option() =
unicode |
anchored |
caseless |
dollar_endonly |
dotall |
extended |
firstline |
multiline |
no_auto_capture |
dupnames |
ungreedy |
{newline, nl_spec()} |
bsr_anycrlf |
bsr_unicode |
no_start_optimize |
ucp |
never_utf
EXPORTS
compile(Regexp) -> {ok, MP} | {error, ErrSpec}
Types:
Regexp = iodata()
MP = mp()
ErrSpec =
{ErrString :: string(), Position :: integer() >= 0}
The same as compile(Regexp,[])
compile(Regexp, Options) -> {ok, MP} | {error, ErrSpec}
Types:
Regexp = iodata() | unicode:charlist()
Options = [Option]
Option = compile_option()
MP = mp()
ErrSpec =
{ErrString :: string(), Position :: integer() >= 0}
Compiles a regular expression, with the syntax described below,
into an internal format to be used later as a parameter to run/2
and run/3.
Compiling the regular expression before matching is useful if
the same expression is to be used in matching against multiple
subjects during the lifetime of the program. Compiling once and
executing many times is far more efficient than compiling each
time one wants to match.
When option unicode is specified, the regular expression is to
be specified as a valid Unicode charlist(), otherwise as any
valid iodata().
Options:
unicode:
The regular expression is specified as a Unicode charlist()
and the resulting regular expression code is to be run
against a valid Unicode charlist() subject. Also consider
option ucp when using Unicode characters.
anchored:
The pattern is forced to be "anchored", that is, it is
constrained to match only at the first matching point in the
string that is searched (the "subject string"). This effect
can also be achieved by appropriate constructs in the
pattern itself.
caseless:
Letters in the pattern match both uppercase and lowercase
letters. It is equivalent to Perl option /i and can be
changed within a pattern by a (?i) option setting. Uppercase
and lowercase letters are defined as in the ISO 8859-1
character set.
dollar_endonly:
A dollar metacharacter in the pattern matches only at the
end of the subject string. Without this option, a dollar
also matches immediately before a newline at the end of the
string (but not before any other newlines). This option is
ignored if option multiline is specified. There is no
equivalent option in Perl, and it cannot be set within a
pattern.
dotall:
A dot in the pattern matches all characters, including those
indicating newline. Without it, a dot does not match when
the current position is at a newline. This option is
equivalent to Perl option /s and it can be changed within a
pattern by a (?s) option setting. A negative class, such as
[^a], always matches newline characters, independent of the
setting of this option.
extended:
Whitespace data characters in the pattern are ignored except
when escaped or inside a character class. Whitespace does
not include character 'vt' (ASCII 11). Characters between an
unescaped # outside a character class and the next newline,
inclusive, are also ignored. This is equivalent to Perl
option /x and can be changed within a pattern by a (?x)
option setting.
With this option, comments inside complicated patterns can
be included. However, notice that this applies only to data
characters. Whitespace characters can never appear within
special character sequences in a pattern, for example within
sequence (?( that introduces a conditional subpattern.
firstline:
An unanchored pattern is required to match before or at the
first newline in the subject string, although the matched
text can continue over the newline.
multiline:
By default, PCRE treats the subject string as consisting of
a single line of characters (even if it contains newlines).
The "start of line" metacharacter (^) matches only at the
start of the string, while the "end of line" metacharacter
($) matches only at the end of the string, or before a
terminating newline (unless option dollar_endonly is
specified). This is the same as in Perl.
When this option is specified, the "start of line" and "end
of line" constructs match immediately following or
immediately before internal newlines in the subject string,
respectively, as well as at the very start and end. This is
equivalent to Perl option /m and can be changed within a
pattern by a (?m) option setting. If there are no newlines
in a subject string, or no occurrences of ^ or $ in a
pattern, setting multiline has no effect.
no_auto_capture:
Disables the use of numbered capturing parentheses in the
pattern. Any opening parenthesis that is not followed by ?
behaves as if it is followed by ?:. Named parentheses can
still be used for capturing (and they acquire numbers in the
usual way). There is no equivalent option in Perl.
dupnames:
Names used to identify capturing subpatterns need not be
unique. This can be helpful for certain types of pattern
when it is known that only one instance of the named
subpattern can ever be matched. More details of named
subpatterns are provided below.
ungreedy:
Inverts the "greediness" of the quantifiers so that they are
not greedy by default, but become greedy if followed by "?".
It is not compatible with Perl. It can also be set by a (?U)
option setting within the pattern.
{newline, NLSpec}:
Overrides the default definition of a newline in the subject
string, which is LF (ASCII 10) in Erlang.
cr:
Newline is indicated by a single character cr (ASCII 13).
lf:
Newline is indicated by a single character LF (ASCII 10),
the default.
crlf:
Newline is indicated by the two-character CRLF (ASCII 13
followed by ASCII 10) sequence.
anycrlf:
Any of the three preceding sequences is to be recognized.
any:
Any of the newline sequences above, and the Unicode
sequences VT (vertical tab, U+000B), FF (formfeed,
U+000C), NEL (next line, U+0085), LS (line separator,
U+2028), and PS (paragraph separator, U+2029).
bsr_anycrlf:
Specifies specifically that \R is to match only the CR, LF,
or CRLF sequences, not the Unicode-specific newline
characters.
bsr_unicode:
Specifies specifically that \R is to match all the Unicode
newline characters (including CRLF, and so on, the default).
no_start_optimize:
Disables optimization that can malfunction if "Special
start-of-pattern items" are present in the regular
expression. A typical example would be when matching
"DEFABC" against "(*COMMIT)ABC", where the start
optimization of PCRE would skip the subject up to "A" and
never realize that the (*COMMIT) instruction is to have made
the matching fail. This option is only relevant if you use
"start-of-pattern items", as discussed in section PCRE
Regular Expression Details.
ucp:
Specifies that Unicode character properties are to be used
when resolving \B, , \D, \d, \S, \s, \W and \w. Without
this flag, only ISO Latin-1 properties are used. Using
Unicode properties hurts performance, but is semantically
correct when working with Unicode characters beyond the ISO
Latin-1 range.
never_utf:
Specifies that the (*UTF) and/or (*UTF8) "start-of-pattern
items" are forbidden. This flag cannot be combined with
option unicode. Useful if ISO Latin-1 patterns from an
external source are to be compiled.
inspect(MP, Item) -> {namelist, [binary()]}
Types:
MP = mp()
Item = namelist
Takes a compiled regular expression and an item, and returns the
relevant data from the regular expression. The only supported
item is namelist, which returns the tuple {namelist,
[binary()]}, containing the names of all (unique) named
subpatterns in the regular expression. For example:
1> {ok,MP} = re:compile("(?<A>A)|(?<B>B)|(?<C>C)").
{ok,{re_pattern,3,0,0,
<<69,82,67,80,119,0,0,0,0,0,0,0,1,0,0,0,255,255,255,255,
255,255,...>>}}
2> re:inspect(MP,namelist).
{namelist,[<<"A">>,<<"B">>,<<"C">>]}
3> {ok,MPD} = re:compile("(?<C>A)|(?<B>B)|(?<C>C)",[dupnames]).
{ok,{re_pattern,3,0,0,
<<69,82,67,80,119,0,0,0,0,0,8,0,1,0,0,0,255,255,255,255,
255,255,...>>}}
4> re:inspect(MPD,namelist).
{namelist,[<<"B">>,<<"C">>]}
Notice in the second example that the duplicate name only occurs
once in the returned list, and that the list is in alphabetical
order regardless of where the names are positioned in the
regular expression. The order of the names is the same as the
order of captured subexpressions if {capture, all_names} is
specified as an option to run/3. You can therefore create a
name-to-value mapping from the result of run/3 like this:
1> {ok,MP} = re:compile("(?<A>A)|(?<B>B)|(?<C>C)").
{ok,{re_pattern,3,0,0,
<<69,82,67,80,119,0,0,0,0,0,0,0,1,0,0,0,255,255,255,255,
255,255,...>>}}
2> {namelist, N} = re:inspect(MP,namelist).
{namelist,[<<"A">>,<<"B">>,<<"C">>]}
3> {match,L} = re:run("AA",MP,[{capture,all_names,binary}]).
{match,[<<"A">>,<<>>,<<>>]}
4> NameMap = lists:zip(N,L).
[{<<"A">>,<<"A">>},{<<"B">>,<<>>},{<<"C">>,<<>>}]
replace(Subject, RE, Replacement) -> iodata() | unicode:charlist()
Types:
Subject = iodata() | unicode:charlist()
RE = mp() | iodata()
Replacement = iodata() | unicode:charlist()
Same as replace(Subject, RE, Replacement, []).
replace(Subject, RE, Replacement, Options) ->
iodata() | unicode:charlist()
Types:
Subject = iodata() | unicode:charlist()
RE = mp() | iodata() | unicode:charlist()
Replacement = iodata() | unicode:charlist()
Options = [Option]
Option =
anchored |
global |
notbol |
noteol |
notempty |
notempty_atstart |
{offset, integer() >= 0} |
{newline, NLSpec} |
bsr_anycrlf |
{match_limit, integer() >= 0} |
{match_limit_recursion, integer() >= 0} |
bsr_unicode |
{return, ReturnType} |
CompileOpt
ReturnType = iodata | list | binary
CompileOpt = compile_option()
NLSpec = cr | crlf | lf | anycrlf | any
Replaces the matched part of the Subject string with the
contents of Replacement.
The permissible options are the same as for run/3, except that
option capture is not allowed. Instead a {return, ReturnType} is
present. The default return type is iodata, constructed in a way
to minimize copying. The iodata result can be used directly in
many I/O operations. If a flat list() is desired, specify
{return, list}. If a binary is desired, specify {return,
binary}.
As in function run/3, an mp() compiled with option unicode
requires Subject to be a Unicode charlist(). If compilation is
done implicitly and the unicode compilation option is specified
to this function, both the regular expression and Subject are to
specified as valid Unicode charlist()s.
The replacement string can contain the special character &,
which inserts the whole matching expression in the result, and
the special sequence \N (where N is an integer > 0), \gN, or
\g{N}, resulting in the subexpression number N, is inserted in
the result. If no subexpression with that number is generated by
the regular expression, nothing is inserted.
To insert an & or a \ in the result, precede it with a \. Notice
that Erlang already gives a special meaning to \ in literal
strings, so a single \ must be written as "\\" and therefore a
double \ as "\\\\".
Example:
re:replace("abcd","c","[&]",[{return,list}]).
gives
"ab[c]d"
while
re:replace("abcd","c","[\\&]",[{return,list}]).
gives
"ab[&]d"
As with run/3, compilation errors raise the badarg exception.
compile/2 can be used to get more information about the error.
run(Subject, RE) -> {match, Captured} | nomatch
Types:
Subject = iodata() | unicode:charlist()
RE = mp() | iodata()
Captured = [CaptureData]
CaptureData = {integer(), integer()}
Same as run(Subject,RE,[]).
run(Subject, RE, Options) ->
{match, Captured} | match | nomatch | {error, ErrType}
Types:
Subject = iodata() | unicode:charlist()
RE = mp() | iodata() | unicode:charlist()
Options = [Option]
Option =
anchored |
global |
notbol |
noteol |
notempty |
notempty_atstart |
report_errors |
{offset, integer() >= 0} |
{match_limit, integer() >= 0} |
{match_limit_recursion, integer() >= 0} |
{newline, NLSpec :: nl_spec()} |
bsr_anycrlf |
bsr_unicode |
{capture, ValueSpec} |
{capture, ValueSpec, Type} |
CompileOpt
Type = index | list | binary
ValueSpec =
all | all_but_first | all_names | first | none |
ValueList
ValueList = [ValueID]
ValueID = integer() | string() | atom()
CompileOpt = compile_option()
See compile/2.
Captured = [CaptureData] | [[CaptureData]]
CaptureData =
{integer(), integer()} | ListConversionData | binary()
ListConversionData =
string() |
{error, string(), binary()} |
{incomplete, string(), binary()}
ErrType =
match_limit | match_limit_recursion | {compile,
CompileErr}
CompileErr =
{ErrString :: string(), Position :: integer() >= 0}
Executes a regular expression matching, and returns
match/{match, Captured} or nomatch. The regular expression can
be specified either as iodata() in which case it is
automatically compiled (as by compile/2) and executed, or as a
precompiled mp() in which case it is executed against the
subject directly.
When compilation is involved, exception badarg is thrown if a
compilation error occurs. Call compile/2 to get information
about the location of the error in the regular expression.
If the regular expression is previously compiled, the option
list can only contain the following options:
* anchored
* {capture, ValueSpec}/{capture, ValueSpec, Type}
* global
* {match_limit, integer() >= 0}
* {match_limit_recursion, integer() >= 0}
* {newline, NLSpec}
* notbol
* notempty
* notempty_atstart
* noteol
* {offset, integer() >= 0}
* report_errors
Otherwise all options valid for function compile/2 are also
allowed. Options allowed both for compilation and execution of a
match, namely anchored and {newline, NLSpec}, affect both the
compilation and execution if present together with a non-
precompiled regular expression.
If the regular expression was previously compiled with option
unicode, Subject is to be provided as a valid Unicode
charlist(), otherwise any iodata() will do. If compilation is
involved and option unicode is specified, both Subject and the
regular expression are to be specified as valid Unicode
charlists().
{capture, ValueSpec}/{capture, ValueSpec, Type} defines what to
return from the function upon successful matching. The capture
tuple can contain both a value specification, telling which of
the captured substrings are to be returned, and a type
specification, telling how captured substrings are to be
returned (as index tuples, lists, or binaries). The options are
described in detail below.
If the capture options describe that no substring capturing is
to be done ({capture, none}), the function returns the single
atom match upon successful matching, otherwise the tuple {match,
ValueList}. Disabling capturing can be done either by specifying
none or an empty list as ValueSpec.
Option report_errors adds the possibility that an error tuple is
returned. The tuple either indicates a matching error
(match_limit or match_limit_recursion), or a compilation error,
where the error tuple has the format {error, {compile,
CompileErr}}. Notice that if option report_errors is not
specified, the function never returns error tuples, but reports
compilation errors as a badarg exception and failed matches
because of exceeded match limits simply as nomatch.
The following options are relevant for execution:
anchored:
Limits run/3 to matching at the first matching position. If
a pattern was compiled with anchored, or turned out to be
anchored by virtue of its contents, it cannot be made
unanchored at matching time, hence there is no unanchored
option.
global:
Implements global (repetitive) search (flag g in Perl). Each
match is returned as a separate list() containing the
specific match and any matching subexpressions (or as
specified by option capture. The Captured part of the return
value is hence a list() of list()s when this option is
specified.
The interaction of option global with a regular expression
that matches an empty string surprises some users. When
option global is specified, run/3 handles empty matches in
the same way as Perl: a zero-length match at any point is
also retried with options [anchored, notempty_atstart]. If
that search gives a result of length > 0, the result is
included. Example:
re:run("cat","(|at)",[global]).
The following matchings are performed:
At offset 0:
The regular expression (|at) first match at the initial
position of string cat, giving the result set
[{0,0},{0,0}] (the second {0,0} is because of the
subexpression marked by the parentheses). As the length of
the match is 0, we do not advance to the next position
yet.
At offset 0 with [anchored, notempty_atstart]:
The search is retried with options [anchored,
notempty_atstart] at the same position, which does not
give any interesting result of longer length, so the
search position is advanced to the next character (a).
At offset 1:
The search results in [{1,0},{1,0}], so this search is
also repeated with the extra options.
At offset 1 with [anchored, notempty_atstart]:
Alternative ab is found and the result is [{1,2},{1,2}].
The result is added to the list of results and the
position in the search string is advanced two steps.
At offset 3:
The search once again matches the empty string, giving
[{3,0},{3,0}].
At offset 1 with [anchored, notempty_atstart]:
This gives no result of length > 0 and we are at the last
position, so the global search is complete.
The result of the call is:
{match,[[{0,0},{0,0}],[{1,0},{1,0}],[{1,2},{1,2}],[{3,0},{3,0}]]}
notempty:
An empty string is not considered to be a valid match if
this option is specified. If alternatives in the pattern
exist, they are tried. If all the alternatives match the
empty string, the entire match fails.
Example:
If the following pattern is applied to a string not
beginning with "a" or "b", it would normally match the empty
string at the start of the subject:
a?b?
With option notempty, this match is invalid, so run/3
searches further into the string for occurrences of "a" or
"b".
notempty_atstart:
Like notempty, except that an empty string match that is not
at the start of the subject is permitted. If the pattern is
anchored, such a match can occur only if the pattern
contains \K.
Perl has no direct equivalent of notempty or
notempty_atstart, but it does make a special case of a
pattern match of the empty string within its split()
function, and when using modifier /g. The Perl behavior can
be emulated after matching a null string by first trying the
match again at the same offset with notempty_atstart and
anchored, and then, if that fails, by advancing the starting
offset (see below) and trying an ordinary match again.
notbol:
Specifies that the first character of the subject string is
not the beginning of a line, so the circumflex metacharacter
is not to match before it. Setting this without multiline
(at compile time) causes circumflex never to match. This
option only affects the behavior of the circumflex
metacharacter. It does not affect \\A.
noteol:
Specifies that the end of the subject string is not the end
of a line, so the dollar metacharacter is not to match it
nor (except in multiline mode) a newline immediately before
it. Setting this without multiline (at compile time) causes
dollar never to match. This option affects only the behavior
of the dollar metacharacter. It does not affect \\Z or \\z.
report_errors:
Gives better control of the error handling in run/3. When
specified, compilation errors (if the regular expression is
not already compiled) and runtime errors are explicitly
returned as an error tuple.
The following are the possible runtime errors:
match_limit:
The PCRE library sets a limit on how many times the
internal match function can be called. Defaults to
10,000,000 in the library compiled for Erlang. If {error,
match_limit} is returned, the execution of the regular
expression has reached this limit. This is normally to be
regarded as a nomatch, which is the default return value
when this occurs, but by specifying report_errors, you are
informed when the match fails because of too many internal
calls.
match_limit_recursion:
This error is very similar to match_limit, but occurs when
the internal match function of PCRE is "recursively"
called more times than the match_limit_recursion limit,
which defaults to 10,000,000 as well. Notice that as long
as the match_limit and match_limit_default values are kept
at the default values, the match_limit_recursion error
cannot occur, as the match_limit error occurs before that
(each recursive call is also a call, but not conversely).
Both limits can however be changed, either by setting
limits directly in the regular expression string (see
section PCRE Regular Eexpression Details) or by specifying
options to run/3.
It is important to understand that what is referred to as
"recursion" when limiting matches is not recursion on the C
stack of the Erlang machine or on the Erlang process stack.
The PCRE version compiled into the Erlang VM uses machine
"heap" memory to store values that must be kept over
recursion in regular expression matches.
{match_limit, integer() >= 0}:
Limits the execution time of a match in an implementation-
specific way. It is described as follows by the PCRE
documentation:
The match_limit field provides a means of preventing PCRE from using
up a vast amount of resources when running patterns that are not going
to match, but which have a very large number of possibilities in their
search trees. The classic example is a pattern that uses nested
unlimited repeats.
Internally, pcre_exec() uses a function called match(), which it calls
repeatedly (sometimes recursively). The limit set by match_limit is
imposed on the number of times this function is called during a match,
which has the effect of limiting the amount of backtracking that can
take place. For patterns that are not anchored, the count restarts
from zero for each position in the subject string.
This means that runaway regular expression matches can fail
faster if the limit is lowered using this option. The
default value 10,000,000 is compiled into the Erlang VM.
Note:
This option does in no way affect the execution of the Erlang
VM in terms of "long running BIFs". run/3 always gives control
back to the scheduler of Erlang processes at intervals that
ensures the real-time properties of the Erlang system.
{match_limit_recursion, integer() >= 0}:
Limits the execution time and memory consumption of a match
in an implementation-specific way, very similar to
match_limit. It is described as follows by the PCRE
documentation:
The match_limit_recursion field is similar to match_limit, but instead
of limiting the total number of times that match() is called, it
limits the depth of recursion. The recursion depth is a smaller number
than the total number of calls, because not all calls to match() are
recursive. This limit is of use only if it is set smaller than
match_limit.
Limiting the recursion depth limits the amount of machine stack that
can be used, or, when PCRE has been compiled to use memory on the heap
instead of the stack, the amount of heap memory that can be used.
The Erlang VM uses a PCRE library where heap memory is used
when regular expression match recursion occurs. This
therefore limits the use of machine heap, not C stack.
Specifying a lower value can result in matches with deep
recursion failing, when they should have matched:
1> re:run("aaaaaaaaaaaaaz","(a+)*z").
{match,[{0,14},{0,13}]}
2> re:run("aaaaaaaaaaaaaz","(a+)*z",[{match_limit_recursion,5}]).
nomatch
3> re:run("aaaaaaaaaaaaaz","(a+)*z",[{match_limit_recursion,5},report_errors]).
{error,match_limit_recursion}
This option and option match_limit are only to be used in
rare cases. Understanding of the PCRE library internals is
recommended before tampering with these limits.
{offset, integer() >= 0}:
Start matching at the offset (position) specified in the
subject string. The offset is zero-based, so that the
default is {offset,0} (all of the subject string).
{newline, NLSpec}:
Overrides the default definition of a newline in the subject
string, which is LF (ASCII 10) in Erlang.
cr:
Newline is indicated by a single character CR (ASCII 13).
lf:
Newline is indicated by a single character LF (ASCII 10),
the default.
crlf:
Newline is indicated by the two-character CRLF (ASCII 13
followed by ASCII 10) sequence.
anycrlf:
Any of the three preceding sequences is be recognized.
any:
Any of the newline sequences above, and the Unicode
sequences VT (vertical tab, U+000B), FF (formfeed,
U+000C), NEL (next line, U+0085), LS (line separator,
U+2028), and PS (paragraph separator, U+2029).
bsr_anycrlf:
Specifies specifically that \R is to match only the CR LF,
or CRLF sequences, not the Unicode-specific newline
characters. (Overrides the compilation option.)
bsr_unicode:
Specifies specifically that \R is to match all the Unicode
newline characters (including CRLF, and so on, the default).
(Overrides the compilation option.)
{capture, ValueSpec}/{capture, ValueSpec, Type}:
Specifies which captured substrings are returned and in what
format. By default, run/3 captures all of the matching part
of the substring and all capturing subpatterns (all of the
pattern is automatically captured). The default return type
is (zero-based) indexes of the captured parts of the string,
specified as {Offset,Length} pairs (the index Type of
capturing).
As an example of the default behavior, the following call
returns, as first and only captured string, the matching
part of the subject ("abcd" in the middle) as an index pair
{3,4}, where character positions are zero-based, just as in
offsets:
re:run("ABCabcdABC","abcd",[]).
The return value of this call is:
{match,[{3,4}]}
Another (and quite common) case is where the regular
expression matches all of the subject:
re:run("ABCabcdABC",".*abcd.*",[]).
Here the return value correspondingly points out all of the
string, beginning at index 0, and it is 10 characters long:
{match,[{0,10}]}
If the regular expression contains capturing subpatterns,
like in:
re:run("ABCabcdABC",".*(abcd).*",[]).
all of the matched subject is captured, as well as the
captured substrings:
{match,[{0,10},{3,4}]}
The complete matching pattern always gives the first return
value in the list and the remaining subpatterns are added in
the order they occurred in the regular expression.
The capture tuple is built up as follows:
ValueSpec:
Specifies which captured (sub)patterns are to be returned.
ValueSpec can either be an atom describing a predefined
set of return values, or a list containing the indexes or
the names of specific subpatterns to return.
The following are the predefined sets of subpatterns:
all:
All captured subpatterns including the complete matching
string. This is the default.
all_names:
All named subpatterns in the regular expression, as if a
list() of all the names in alphabetical order was
specified. The list of all names can also be retrieved
with inspect/2.
first:
Only the first captured subpattern, which is always the
complete matching part of the subject. All explicitly
captured subpatterns are discarded.
all_but_first:
All but the first matching subpattern, that is, all
explicitly captured subpatterns, but not the complete
matching part of the subject string. This is useful if
the regular expression as a whole matches a large part
of the subject, but the part you are interested in is in
an explicitly captured subpattern. If the return type is
list or binary, not returning subpatterns you are not
interested in is a good way to optimize.
none:
Returns no matching subpatterns, gives the single atom
match as the return value of the function when matching
successfully instead of the {match, list()} return.
Specifying an empty list gives the same behavior.
The value list is a list of indexes for the subpatterns to
return, where index 0 is for all of the pattern, and 1 is
for the first explicit capturing subpattern in the regular
expression, and so on. When using named captured
subpatterns (see below) in the regular expression, one can
use atom()s or string()s to specify the subpatterns to be
returned. For example, consider the regular expression:
".*(abcd).*"
matched against string "ABCabcdABC", capturing only the
"abcd" part (the first explicit subpattern):
re:run("ABCabcdABC",".*(abcd).*",[{capture,[1]}]).
The call gives the following result, as the first
explicitly captured subpattern is "(abcd)", matching
"abcd" in the subject, at (zero-based) position 3, of
length 4:
{match,[{3,4}]}
Consider the same regular expression, but with the
subpattern explicitly named 'FOO':
".*(?<FOO>abcd).*"
With this expression, we could still give the index of the
subpattern with the following call:
re:run("ABCabcdABC",".*(?<FOO>abcd).*",[{capture,[1]}]).
giving the same result as before. But, as the subpattern
is named, we can also specify its name in the value list:
re:run("ABCabcdABC",".*(?<FOO>abcd).*",[{capture,['FOO']}]).
This would give the same result as the earlier examples,
namely:
{match,[{3,4}]}
The values list can specify indexes or names not present
in the regular expression, in which case the return values
vary depending on the type. If the type is index, the
tuple {-1,0} is returned for values with no corresponding
subpattern in the regular expression, but for the other
types (binary and list), the values are the empty binary
or list, respectively.
Type:
Optionally specifies how captured substrings are to be
returned. If omitted, the default of index is used.
Type can be one of the following:
index:
Returns captured substrings as pairs of byte indexes
into the subject string and length of the matching
string in the subject (as if the subject string was
flattened with erlang:iolist_to_binary/1 or
unicode:characters_to_binary/2 before matching). Notice
that option unicode results in byte-oriented indexes in
a (possibly virtual) UTF-8 encoded binary. A byte index
tuple {0,2} can therefore represent one or two
characters when unicode is in effect. This can seem
counter-intuitive, but has been deemed the most
effective and useful way to do it. To return lists
instead can result in simpler code if that is desired.
This return type is the default.
list:
Returns matching substrings as lists of characters
(Erlang string()s). It option unicode is used in
combination with the \C sequence in the regular
expression, a captured subpattern can contain bytes that
are not valid UTF-8 (\C matches bytes regardless of
character encoding). In that case the list capturing can
result in the same types of tuples that
unicode:characters_to_list/2 can return, namely three-
tuples with tag incomplete or error, the successfully
converted characters and the invalid UTF-8 tail of the
conversion as a binary. The best strategy is to avoid
using the \C sequence when capturing lists.
binary:
Returns matching substrings as binaries. If option
unicode is used, these binaries are in UTF-8. If the \C
sequence is used together with unicode, the binaries can
be invalid UTF-8.
In general, subpatterns that were not assigned a value in
the match are returned as the tuple {-1,0} when type is
index. Unassigned subpatterns are returned as the empty
binary or list, respectively, for other return types.
Consider the following regular expression:
".*((?<FOO>abdd)|a(..d)).*"
There are three explicitly capturing subpatterns, where the
opening parenthesis position determines the order in the
result, hence ((?<FOO>abdd)|a(..d)) is subpattern index 1,
(?<FOO>abdd) is subpattern index 2, and (..d) is subpattern
index 3. When matched against the following string:
"ABCabcdABC"
the subpattern at index 2 does not match, as "abdd" is not
present in the string, but the complete pattern matches
(because of the alternative a(..d)). The subpattern at index
2 is therefore unassigned and the default return value is:
{match,[{0,10},{3,4},{-1,0},{4,3}]}
Setting the capture Type to binary gives:
{match,[<<"ABCabcdABC">>,<<"abcd">>,<<>>,<<"bcd">>]}
Here the empty binary (<<>>) represents the unassigned
subpattern. In the binary case, some information about the
matching is therefore lost, as <<>> can also be an empty
string captured.
If differentiation between empty matches and non-existing
subpatterns is necessary, use the type index and do the
conversion to the final type in Erlang code.
When option global is speciified, the capture specification
affects each match separately, so that:
re:run("cacb","c(a|b)",[global,{capture,[1],list}]).
gives
{match,[["a"],["b"]]}
For a descriptions of options only affecting the compilation
step, see compile/2.
split(Subject, RE) -> SplitList
Types:
Subject = iodata() | unicode:charlist()
RE = mp() | iodata()
SplitList = [iodata() | unicode:charlist()]
Same as split(Subject, RE, []).
split(Subject, RE, Options) -> SplitList
Types:
Subject = iodata() | unicode:charlist()
RE = mp() | iodata() | unicode:charlist()
Options = [Option]
Option =
anchored |
notbol |
noteol |
notempty |
notempty_atstart |
{offset, integer() >= 0} |
{newline, nl_spec()} |
{match_limit, integer() >= 0} |
{match_limit_recursion, integer() >= 0} |
bsr_anycrlf |
bsr_unicode |
{return, ReturnType} |
{parts, NumParts} |
group |
trim |
CompileOpt
NumParts = integer() >= 0 | infinity
ReturnType = iodata | list | binary
CompileOpt = compile_option()
See compile/2.
SplitList = [RetData] | [GroupedRetData]
GroupedRetData = [RetData]
RetData = iodata() | unicode:charlist() | binary() | list()
Splits the input into parts by finding tokens according to the
regular expression supplied. The splitting is basically done by
running a global regular expression match and dividing the
initial string wherever a match occurs. The matching part of the
string is removed from the output.
As in run/3, an mp() compiled with option unicode requires
Subject to be a Unicode charlist(). If compilation is done
implicitly and the unicode compilation option is specified to
this function, both the regular expression and Subject are to be
specified as valid Unicode charlist()s.
The result is given as a list of "strings", the preferred data
type specified in option return (default iodata).
If subexpressions are specified in the regular expression, the
matching subexpressions are returned in the resulting list as
well. For example:
re:split("Erlang","[ln]",[{return,list}]).
gives
["Er","a","g"]
while
re:split("Erlang","([ln])",[{return,list}]).
gives
["Er","l","a","n","g"]
The text matching the subexpression (marked by the parentheses
in the regular expression) is inserted in the result list where
it was found. This means that concatenating the result of a
split where the whole regular expression is a single
subexpression (as in the last example) always results in the
original string.
As there is no matching subexpression for the last part in the
example (the "g"), nothing is inserted after that. To make the
group of strings and the parts matching the subexpressions more
obvious, one can use option group, which groups together the
part of the subject string with the parts matching the
subexpressions when the string was split:
re:split("Erlang","([ln])",[{return,list},group]).
gives
[["Er","l"],["a","n"],["g"]]
Here the regular expression first matched the "l", causing "Er"
to be the first part in the result. When the regular expression
matched, the (only) subexpression was bound to the "l", so the
"l" is inserted in the group together with "Er". The next match
is of the "n", making "a" the next part to be returned. As the
subexpression is bound to substring "n" in this case, the "n" is
inserted into this group. The last group consists of the
remaining string, as no more matches are found.
By default, all parts of the string, including the empty
strings, are returned from the function, for example:
re:split("Erlang","[lg]",[{return,list}]).
gives
["Er","an",[]]
as the matching of the "g" in the end of the string leaves an
empty rest, which is also returned. This behavior differs from
the default behavior of the split function in Perl, where empty
strings at the end are by default removed. To get the "trimming"
default behavior of Perl, specify trim as an option:
re:split("Erlang","[lg]",[{return,list},trim]).
gives
["Er","an"]
The "trim" option says; "give me as many parts as possible
except the empty ones", which sometimes can be useful. You can
also specify how many parts you want, by specifying {parts,N}:
re:split("Erlang","[lg]",[{return,list},{parts,2}]).
gives
["Er","ang"]
Notice that the last part is "ang", not "an", as splitting was
specified into two parts, and the splitting stops when enough
parts are given, which is why the result differs from that of
trim.
More than three parts are not possible with this indata, so
re:split("Erlang","[lg]",[{return,list},{parts,4}]).
gives the same result as the default, which is to be viewed as
"an infinite number of parts".
Specifying 0 as the number of parts gives the same effect as
option trim. If subexpressions are captured, empty
subexpressions matched at the end are also stripped from the
result if trim or {parts,0} is specified.
The trim behavior corresponds exactly to the Perl default.
{parts,N}, where N is a positive integer, corresponds exactly to
the Perl behavior with a positive numerical third parameter. The
default behavior of split/3 corresponds to the Perl behavior
when a negative integer is specified as the third parameter for
the Perl routine.
Summary of options not previously described for function run/3:
{return,ReturnType}:
Specifies how the parts of the original string are presented
in the result list. Valid types:
iodata:
The variant of iodata() that gives the least copying of
data with the current implementation (often a binary, but
do not depend on it).
binary:
All parts returned as binaries.
list:
All parts returned as lists of characters ("strings").
group:
Groups together the part of the string with the parts of the
string matching the subexpressions of the regular
expression.
The return value from the function is in this case a list()
of list()s. Each sublist begins with the string picked out
of the subject string, followed by the parts matching each
of the subexpressions in order of occurrence in the regular
expression.
{parts,N}:
Specifies the number of parts the subject string is to be
split into.
The number of parts is to be a positive integer for a
specific maximum number of parts, and infinity for the
maximum number of parts possible (the default). Specifying
{parts,0} gives as many parts as possible disregarding empty
parts at the end, the same as specifying trim.
trim:
Specifies that empty parts at the end of the result list are
to be disregarded. The same as specifying {parts,0}. This
corresponds to the default behavior of the split built-in
function in Perl.
PERL-LIKE REGULAR EXPRESSION SYNTAX
The following sections contain reference material for the regular expressions used by this module. The information is based on the PCRE documentation, with changes where this module behaves differently to the PCRE library.
PCRE REGULAR EXPRESSION DETAILS
The syntax and semantics of the regular expressions supported by PCRE
are described in detail in the following sections. Perl's regular
expressions are described in its own documentation, and regular
expressions in general are covered in many books, some with copious
examples. Jeffrey Friedl's "Mastering Regular Expressions", published
by O'Reilly, covers regular expressions in great detail. This
description of the PCRE regular expressions is intended as reference
material.
The reference material is divided into the following sections:
* Special Start-of-Pattern Items
* Characters and Metacharacters
* Backslash
* Circumflex and Dollar
* Full Stop (Period, Dot) and \N
* Matching a Single Data Unit
* Square Brackets and Character Classes
* Posix Character Classes
* Vertical Bar
* Internal Option Setting
* Subpatterns
* Duplicate Subpattern Numbers
* Named Subpatterns
* Repetition
* Atomic Grouping and Possessive Quantifiers
* Back References
* Assertions
* Conditional Subpatterns
* Comments
* Recursive Patterns
* Subpatterns as Subroutines
* Oniguruma Subroutine Syntax
* Backtracking Control
SPECIAL START-OF-PATTERN ITEMS
Some options that can be passed to compile/2 can also be set by special
items at the start of a pattern. These are not Perl-compatible, but are
provided to make these options accessible to pattern writers who are
not able to change the program that processes the pattern. Any number
of these items can appear, but they must all be together right at the
start of the pattern string, and the letters must be in upper case.
UTF Support
Unicode support is basically UTF-8 based. To use Unicode characters,
you either call compile/2 or run/3 with option unicode, or the pattern
must start with one of these special sequences:
(*UTF8)
(*UTF)
Both options give the same effect, the input string is interpreted as
UTF-8. Notice that with these instructions, the automatic conversion of
lists to UTF-8 is not performed by the re functions. Therefore, using
these sequences is not recommended. Add option unicode when running
compile/2 instead.
Some applications that allow their users to supply patterns can wish to
restrict them to non-UTF data for security reasons. If option never_utf
is set at compile time, (*UTF), and so on, are not allowed, and their
appearance causes an error.
Unicode Property Support
The following is another special sequence that can appear at the start
of a pattern:
(*UCP)
This has the same effect as setting option ucp: it causes sequences
such as \d and \w to use Unicode properties to determine character
types, instead of recognizing only characters with codes < 256 through
a lookup table.
Disabling Startup Optimizations
If a pattern starts with (*NO_START_OPT), it has the same effect as
setting option no_start_optimize at compile time.
Newline Conventions
PCRE supports five conventions for indicating line breaks in strings: a
single CR (carriage return) character, a single LF (line feed)
character, the two-character sequence CRLF, any of the three preceding,
and any Unicode newline sequence.
A newline convention can also be specified by starting a pattern string
with one of the following five sequences:
(*CR):
Carriage return
(*LF):
Line feed
(*CRLF):
>Carriage return followed by line feed
(*ANYCRLF):
Any of the three above
(*ANY):
All Unicode newline sequences
These override the default and the options specified to compile/2. For
example, the following pattern changes the convention to CR:
(*CR)a.b
This pattern matches a\nb, as LF is no longer a newline. If more than
one of them is present, the last one is used.
The newline convention affects where the circumflex and dollar
assertions are true. It also affects the interpretation of the dot
metacharacter when dotall is not set, and the behavior of \N. However,
it does not affect what the \R escape sequence matches. By default,
this is any Unicode newline sequence, for Perl compatibility. However,
this can be changed; see the description of \R in section Newline
Sequences. A change of the \R setting can be combined with a change of
the newline convention.
Setting Match and Recursion Limits
The caller of run/3 can set a limit on the number of times the internal
match() function is called and on the maximum depth of recursive calls.
These facilities are provided to catch runaway matches that are
provoked by patterns with huge matching trees (a typical example is a
pattern with nested unlimited repeats) and to avoid running out of
system stack by too much recursion. When one of these limits is
reached, pcre_exec() gives an error return. The limits can also be set
by items at the start of the pattern of the following forms:
(*LIMIT_MATCH=d)
(*LIMIT_RECURSION=d)
Here d is any number of decimal digits. However, the value of the
setting must be less than the value set by the caller of run/3 for it
to have any effect. That is, the pattern writer can lower the limit set
by the programmer, but not raise it. If there is more than one setting
of one of these limits, the lower value is used.
The default value for both the limits is 10,000,000 in the Erlang VM.
Notice that the recursion limit does not affect the stack depth of the
VM, as PCRE for Erlang is compiled in such a way that the match
function never does recursion on the C stack.
CHARACTERS AND METACHARACTERS
A regular expression is a pattern that is matched against a subject
string from left to right. Most characters stand for themselves in a
pattern and match the corresponding characters in the subject. As a
trivial example, the following pattern matches a portion of a subject
string that is identical to itself:
The quick brown fox
When caseless matching is specified (option caseless), letters are
matched independently of case.
The power of regular expressions comes from the ability to include
alternatives and repetitions in the pattern. These are encoded in the
pattern by the use of metacharacters, which do not stand for themselves
but instead are interpreted in some special way.
Two sets of metacharacters exist: those that are recognized anywhere in
the pattern except within square brackets, and those that are
recognized within square brackets. Outside square brackets, the
metacharacters are as follows:
\:
General escape character with many uses
^:
Assert start of string (or line, in multiline mode)
$:
Assert end of string (or line, in multiline mode)
.:
Match any character except newline (by default)
[:
Start character class definition
|:
Start of alternative branch
(:
Start subpattern
):
End subpattern
?:
Extends the meaning of (, also 0 or 1 quantifier, also quantifier
minimizer
*:
0 or more quantifiers
+:
1 or more quantifier, also "possessive quantifier"
{:
Start min/max quantifier
Part of a pattern within square brackets is called a "character class".
The following are the only metacharacters in a character class:
\:
General escape character
^:
Negate the class, but only if the first character
-:
Indicates character range
[:
Posix character class (only if followed by Posix syntax)
]:
Terminates the character class
The following sections describe the use of each metacharacter.
BACKSLASH
The backslash character has many uses. First, if it is followed by a
character that is not a number or a letter, it takes away any special
meaning that a character can have. This use of backslash as an escape
character applies both inside and outside character classes.
For example, if you want to match a * character, you write \* in the
pattern. This escaping action applies if the following character would
otherwise be interpreted as a metacharacter, so it is always safe to
precede a non-alphanumeric with backslash to specify that it stands for
itself. In particular, if you want to match a backslash, write \\.
In unicode mode, only ASCII numbers and letters have any special
meaning after a backslash. All other characters (in particular, those
whose code points are > 127) are treated as literals.
If a pattern is compiled with option extended, whitespace in the
pattern (other than in a character class) and characters between a #
outside a character class and the next newline are ignored. An escaping
backslash can be used to include a whitespace or # character as part of
the pattern.
To remove the special meaning from a sequence of characters, put them
between \Q and \E. This is different from Perl in that $ and @ are
handled as literals in \Q...\E sequences in PCRE, while $ and @ cause
variable interpolation in Perl. Notice the following examples:
Pattern PCRE matches Perl matches
\Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
\Qabc\$xyz\E abc\$xyz abc\$xyz
\Qabc\E\$\Qxyz\E abc$xyz abc$xyz
The \Q...\E sequence is recognized both inside and outside character
classes. An isolated \E that is not preceded by \Q is ignored. If \Q is
not followed by \E later in the pattern, the literal interpretation
continues to the end of the pattern (that is, \E is assumed at the
end). If the isolated \Q is inside a character class, this causes an
error, as the character class is not terminated.
Non-Printing Characters
A second use of backslash provides a way of encoding non-printing
characters in patterns in a visible manner. There is no restriction on
the appearance of non-printing characters, apart from the binary zero
that terminates a pattern. When a pattern is prepared by text editing,
it is often easier to use one of the following escape sequences than
the binary character it represents:
:
Alarm, that is, the BEL character (hex 07)
\cx:
"Control-x", where x is any ASCII character
\e:
Escape (hex 1B)
\f:
Form feed (hex 0C)
\n:
Line feed (hex 0A)
\r:
Carriage return (hex 0D)
\t:
Tab (hex 09)
\ddd:
Character with octal code ddd, or back reference
\xhh:
Character with hex code hh
\x{hhh..}:
Character with hex code hhh..
The precise effect of \cx on ASCII characters is as follows: if x is a
lowercase letter, it is converted to upper case. Then bit 6 of the
character (hex 40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A
(A is 41, Z is 5A), but \c{ becomes hex 3B ({ is 7B), and \c; becomes
hex 7B (; is 3B). If the data item (byte or 16-bit value) following \c
has a value > 127, a compile-time error occurs. This locks out non-
ASCII characters in all modes.
The \c facility was designed for use with ASCII characters, but with
the extension to Unicode it is even less useful than it once was.
By default, after \x, from zero to two hexadecimal digits are read
(letters can be in upper or lower case). Any number of hexadecimal
digits can appear between \x{ and }, but the character code is
constrained as follows:
8-bit non-Unicode mode:
< 0x100
8-bit UTF-8 mode:
< 0x10ffff and a valid code point
Invalid Unicode code points are the range 0xd800 to 0xdfff (the so-
called "surrogate" code points), and 0xffef.
If characters other than hexadecimal digits appear between \x{ and },
or if there is no terminating }, this form of escape is not recognized.
Instead, the initial \x is interpreted as a basic hexadecimal escape,
with no following digits, giving a character whose value is zero.
Characters whose value is < 256 can be defined by either of the two
syntaxes for \x. There is no difference in the way they are handled.
For example, \xdc is the same as \x{dc}.
After \0 up to two further octal digits are read. If there are fewer
than two digits, only those that are present are used. Thus the
sequence \0\x\07 specifies two binary zeros followed by a BEL character
(code value 7). Ensure to supply two digits after the initial zero if
the pattern character that follows is itself an octal digit.
The handling of a backslash followed by a digit other than 0 is
complicated. Outside a character class, PCRE reads it and any following
digits as a decimal number. If the number is < 10, or if there have
been at least that many previous capturing left parentheses in the
expression, the entire sequence is taken as a back reference. A
description of how this works is provided later, following the
discussion of parenthesized subpatterns.
Inside a character class, or if the decimal number is > 9 and there
have not been that many capturing subpatterns, PCRE re-reads up to
three octal digits following the backslash, and uses them to generate a
data character. Any subsequent digits stand for themselves. The value
of the character is constrained in the same way as characters specified
in hexadecimal. For example:
\040:
Another way of writing an ASCII space
\40:
The same, provided there are < 40 previous capturing subpatterns
\7:
Always a back reference
\11:
Can be a back reference, or another way of writing a tab
\011:
Always a tab
\0113:
A tab followed by character "3"
\113:
Can be a back reference, otherwise the character with octal code
113
\377:
Can be a back reference, otherwise value 255 (decimal)
\81:
Either a back reference, or a binary zero followed by the two
characters "8" and "1"
Notice that octal values >= 100 must not be introduced by a leading
zero, as no more than three octal digits are ever read.
All the sequences that define a single character value can be used both
inside and outside character classes. Also, inside a character class,
is interpreted as the backspace character (hex 08).
\N is not allowed in a character class. \B, \R, and \X are not special
inside a character class. Like other unrecognized escape sequences,
they are treated as the literal characters "B", "R", and "X". Outside a
character class, these sequences have different meanings.
Unsupported Escape Sequences
In Perl, the sequences \l, \L, \u, and \U are recognized by its string
handler and used to modify the case of following characters. PCRE does
not support these escape sequences.
Absolute and Relative Back References
The sequence \g followed by an unsigned or a negative number,
optionally enclosed in braces, is an absolute or relative back
reference. A named back reference can be coded as \g{name}. Back
references are discussed later, following the discussion of
parenthesized subpatterns.
Absolute and Relative Subroutine Calls
For compatibility with Oniguruma, the non-Perl syntax \g followed by a
name or a number enclosed either in angle brackets or single quotes, is
alternative syntax for referencing a subpattern as a "subroutine".
Details are discussed later. Notice that \g{...} (Perl syntax) and
\g<...> (Oniguruma syntax) are not synonymous. The former is a back
reference and the latter is a subroutine call.
Generic Character Types
Another use of backslash is for specifying generic character types:
\d:
Any decimal digit
\D:
Any character that is not a decimal digit
\h:
Any horizontal whitespace character
\H:
Any character that is not a horizontal whitespace character
\s:
Any whitespace character
\S:
Any character that is not a whitespace character
\v:
Any vertical whitespace character
\V:
Any character that is not a vertical whitespace character
\w:
Any "word" character
\W:
Any "non-word" character
There is also the single sequence \N, which matches a non-newline
character. This is the same as the "." metacharacter when dotall is not
set. Perl also uses \N to match characters by name, but PCRE does not
support this.
Each pair of lowercase and uppercase escape sequences partitions the
complete set of characters into two disjoint sets. Any given character
matches one, and only one, of each pair. The sequences can appear both
inside and outside character classes. They each match one character of
the appropriate type. If the current matching point is at the end of
the subject string, all fail, as there is no character to match.
For compatibility with Perl, \s does not match the VT character (code
11). This makes it different from the Posix "space" class. The \s
characters are HT (9), LF (10), FF (12), CR (13), and space (32). If
"use locale;" is included in a Perl script, \s can match the VT
character. In PCRE, it never does.
A "word" character is an underscore or any character that is a letter
or a digit. By default, the definition of letters and digits is
controlled by the PCRE low-valued character tables, in Erlang's case
(and without option unicode), the ISO Latin-1 character set.
By default, in unicode mode, characters with values > 255, that is, all
characters outside the ISO Latin-1 character set, never match \d, \s,
or \w, and always match \D, \S, and \W. These sequences retain their
original meanings from before UTF support was available, mainly for
efficiency reasons. However, if option ucp is set, the behavior is
changed so that Unicode properties are used to determine character
types, as follows:
\d:
Any character that \p{Nd} matches (decimal digit)
\s:
Any character that \p{Z} matches, plus HT, LF, FF, CR
\w:
Any character that \p{L} or \p{N} matches, plus underscore
The uppercase escapes match the inverse sets of characters. Notice that
\d matches only decimal digits, while \w matches any Unicode digit, any
Unicode letter, and underscore. Notice also that ucp affects and \B,
as they are defined in terms of \w and \W. Matching these sequences is
noticeably slower when ucp is set.
The sequences \h, \H, \v, and \V are features that were added to Perl
in release 5.10. In contrast to the other sequences, which match only
ASCII characters by default, these always match certain high-valued
code points, regardless if ucp is set.
The following are the horizontal space characters:
U+0009:
Horizontal tab (HT)
U+0020:
Space
U+00A0:
Non-break space
U+1680:
Ogham space mark
U+180E:
Mongolian vowel separator
U+2000:
En quad
U+2001:
Em quad
U+2002:
En space
U+2003:
Em space
U+2004:
Three-per-em space
U+2005:
Four-per-em space
U+2006:
Six-per-em space
U+2007:
Figure space
U+2008:
Punctuation space
U+2009:
Thin space
U+200A:
Hair space
U+202F:
Narrow no-break space
U+205F:
Medium mathematical space
U+3000:
Ideographic space
The following are the vertical space characters:
U+000A:
Line feed (LF)
U+000B:
Vertical tab (VT)
U+000C:
Form feed (FF)
U+000D:
Carriage return (CR)
U+0085:
Next line (NEL)
U+2028:
Line separator
U+2029:
Paragraph separator
In 8-bit, non-UTF-8 mode, only the characters with code points < 256
are relevant.
Newline Sequences
Outside a character class, by default, the escape sequence \R matches
any Unicode newline sequence. In non-UTF-8 mode, \R is equivalent to
the following:
(?>\r\n|\n|\x0b|\f|\r|\x85)
This is an example of an "atomic group", details are provided below.
This particular group matches either the two-character sequence CR
followed by LF, or one of the single characters LF (line feed, U+000A),
VT (vertical tab, U+000B), FF (form feed, U+000C), CR (carriage return,
U+000D), or NEL (next line, U+0085). The two-character sequence is
treated as a single unit that cannot be split.
In Unicode mode, two more characters whose code points are > 255 are
added: LS (line separator, U+2028) and PS (paragraph separator,
U+2029). Unicode character property support is not needed for these
characters to be recognized.
\R can be restricted to match only CR, LF, or CRLF (instead of the
complete set of Unicode line endings) by setting option bsr_anycrlf
either at compile time or when the pattern is matched. (BSR is an
acronym for "backslash R".) This can be made the default when PCRE is
built; if so, the other behavior can be requested through option
bsr_unicode. These settings can also be specified by starting a pattern
string with one of the following sequences:
(*BSR_ANYCRLF):
CR, LF, or CRLF only
(*BSR_UNICODE):
Any Unicode newline sequence
These override the default and the options specified to the compiling
function, but they can themselves be overridden by options specified to
a matching function. Notice that these special settings, which are not
Perl-compatible, are recognized only at the very start of a pattern,
and that they must be in upper case. If more than one of them is
present, the last one is used. They can be combined with a change of
newline convention; for example, a pattern can start with:
(*ANY)(*BSR_ANYCRLF)
They can also be combined with the (*UTF8), (*UTF), or (*UCP) special
sequences. Inside a character class, \R is treated as an unrecognized
escape sequence, and so matches the letter "R" by default.
Unicode Character Properties
Three more escape sequences that match characters with specific
properties are available. When in 8-bit non-UTF-8 mode, these sequences
are limited to testing characters whose code points are < 256, but they
do work in this mode. The following are the extra escape sequences:
\p{xx}:
A character with property xx
\P{xx}:
A character without property xx
\X:
A Unicode extended grapheme cluster
The property names represented by xx above are limited to the Unicode
script names, the general category properties, "Any", which matches any
character (including newline), and some special PCRE properties
(described in the next section). Other Perl properties, such as
"InMusicalSymbols", are currently not supported by PCRE. Notice that
\P{Any} does not match any characters and always causes a match
failure.
Sets of Unicode characters are defined as belonging to certain scripts.
A character from one of these sets can be matched using a script name,
for example:
\p{Greek} \P{Han}
Those that are not part of an identified script are lumped together as
"Common". The following is the current list of scripts:
* Arabic
* Armenian
* Avestan
* Balinese
* Bamum
* Batak
* Bengali
* Bopomofo
* Braille
* Buginese
* Buhid
* Canadian_Aboriginal
* Carian
* Chakma
* Cham
* Cherokee
* Common
* Coptic
* Cuneiform
* Cypriot
* Cyrillic
* Deseret
* Devanagari
* Egyptian_Hieroglyphs
* Ethiopic
* Georgian
* Glagolitic
* Gothic
* Greek
* Gujarati
* Gurmukhi
* Han
* Hangul
* Hanunoo
* Hebrew
* Hiragana
* Imperial_Aramaic
* Inherited
* Inscriptional_Pahlavi
* Inscriptional_Parthian
* Javanese
* Kaithi
* Kannada
* Katakana
* Kayah_Li
* Kharoshthi
* Khmer
* Lao
* Latin
* Lepcha
* Limbu
* Linear_B
* Lisu
* Lycian
* Lydian
* Malayalam
* Mandaic
* Meetei_Mayek
* Meroitic_Cursive
* Meroitic_Hieroglyphs
* Miao
* Mongolian
* Myanmar
* New_Tai_Lue
* Nko
* Ogham
* Old_Italic
* Old_Persian
* Oriya
* Old_South_Arabian
* Old_Turkic
* Ol_Chiki
* Osmanya
* Phags_Pa
* Phoenician
* Rejang
* Runic
* Samaritan
* Saurashtra
* Sharada
* Shavian
* Sinhala
* Sora_Sompeng
* Sundanese
* Syloti_Nagri
* Syriac
* Tagalog
* Tagbanwa
* Tai_Le
* Tai_Tham
* Tai_Viet
* Takri
* Tamil
* Telugu
* Thaana
* Thai
* Tibetan
* Tifinagh
* Ugaritic
* Vai
* Yi
Each character has exactly one Unicode general category property,
specified by a two-letter acronym. For compatibility with Perl,
negation can be specified by including a circumflex between the opening
brace and the property name. For example, \p{^Lu} is the same as
\P{Lu}.
If only one letter is specified with \p or \P, it includes all the
general category properties that start with that letter. In this case,
in the absence of negation, the curly brackets in the escape sequence
are optional. The following two examples have the same effect:
\p{L}
\pL
The following general category property codes are supported:
C:
Other
Cc:
Control
Cf:
Format
Cn:
Unassigned
Co:
Private use
Cs:
Surrogate
L:
Letter
Ll:
Lowercase letter
Lm:
Modifier letter
Lo:
Other letter
Lt:
Title case letter
Lu:
Uppercase letter
M:
Mark
Mc:
Spacing mark
Me:
Enclosing mark
Mn:
Non-spacing mark
N:
Number
Nd:
Decimal number
Nl:
Letter number
No:
Other number
P:
Punctuation
Pc:
Connector punctuation
Pd:
Dash punctuation
Pe:
Close punctuation
Pf:
Final punctuation
Pi:
Initial punctuation
Po:
Other punctuation
Ps:
Open punctuation
S:
Symbol
Sc:
Currency symbol
Sk:
Modifier symbol
Sm:
Mathematical symbol
So:
Other symbol
Z:
Separator
Zl:
Line separator
Zp:
Paragraph separator
Zs:
Space separator
The special property L& is also supported. It matches a character that
has the Lu, Ll, or Lt property, that is, a letter that is not
classified as a modifier or "other".
The Cs (Surrogate) property applies only to characters in the range
U+D800 to U+DFFF. Such characters are invalid in Unicode strings and so
cannot be tested by PCRE. Perl does not support the Cs property.
The long synonyms for property names supported by Perl (such as
\p{Letter}) are not supported by PCRE. It is not permitted to prefix
any of these properties with "Is".
No character in the Unicode table has the Cn (unassigned) property.
This property is instead assumed for any code point that is not in the
Unicode table.
Specifying caseless matching does not affect these escape sequences.
For example, \p{Lu} always matches only uppercase letters. This is
different from the behavior of current versions of Perl.
Matching characters by Unicode property is not fast, as PCRE must do a
multistage table lookup to find a character property. That is why the
traditional escape sequences such as \d and \w do not use Unicode
properties in PCRE by default. However, you can make them do so by
setting option ucp or by starting the pattern with (*UCP).
Extended Grapheme Clusters
The \X escape matches any number of Unicode characters that form an
"extended grapheme cluster", and treats the sequence as an atomic group
(see below). Up to and including release 8.31, PCRE matched an earlier,
simpler definition that was equivalent to (?>\PM\pM*). That is, it
matched a character without the "mark" property, followed by zero or
more characters with the "mark" property. Characters with the "mark"
property are typically non-spacing accents that affect the preceding
character.
This simple definition was extended in Unicode to include more
complicated kinds of composite character by giving each character a
grapheme breaking property, and creating rules that use these
properties to define the boundaries of extended grapheme clusters. In
PCRE releases later than 8.31, \X matches one of these clusters.
\X always matches at least one character. Then it decides whether to
add more characters according to the following rules for ending a
cluster:
* End at the end of the subject string.
* Do not end between CR and LF; otherwise end after any control
character.
* Do not break Hangul (a Korean script) syllable sequences. Hangul
characters are of five types: L, V, T, LV, and LVT. An L character
can be followed by an L, V, LV, or LVT character. An LV or V
character can be followed by a V or T character. An LVT or T
character can be followed only by a T character.
* Do not end before extending characters or spacing marks. Characters
with the "mark" property always have the "extend" grapheme breaking
property.
* Do not end after prepend characters.
* Otherwise, end the cluster.
PCRE Additional Properties
In addition to the standard Unicode properties described earlier, PCRE
supports four more that make it possible to convert traditional escape
sequences, such as \w and \s, and Posix character classes to use
Unicode properties. PCRE uses these non-standard, non-Perl properties
internally when PCRE_UCP is set. However, they can also be used
explicitly. The properties are as follows:
Xan:
Any alphanumeric character. Matches characters that have either the
L (letter) or the N (number) property.
Xps:
Any Posix space character. Matches the characters tab, line feed,
vertical tab, form feed, carriage return, and any other character
that has the Z (separator) property.
Xsp:
Any Perl space character. Matches the same as Xps, except that
vertical tab is excluded.
Xwd:
Any Perl "word" character. Matches the same characters as Xan, plus
underscore.
There is another non-standard property, Xuc, which matches any
character that can be represented by a Universal Character Name in C++
and other programming languages. These are the characters $, @, `
(grave accent), and all characters with Unicode code points >= U+00A0,
except for the surrogates U+D800 to U+DFFF. Notice that most base
(ASCII) characters are excluded. (Universal Character Names are of the
form \uHHHH or \UHHHHHHHH, where H is a hexadecimal digit. Notice that
the Xuc property does not match these sequences but the characters that
they represent.)
Resetting the Match Start
The escape sequence \K causes any previously matched characters not to
be included in the final matched sequence. For example, the following
pattern matches "foobar", but reports that it has matched "bar":
foo\Kbar
This feature is similar to a lookbehind assertion (described below).
However, in this case, the part of the subject before the real match
does not have to be of fixed length, as lookbehind assertions do. The
use of \K does not interfere with the setting of captured substrings.
For example, when the following pattern matches "foobar", the first
substring is still set to "foo":
(foo)\Kbar
Perl documents that the use of \K within assertions is "not well
defined". In PCRE, \K is acted upon when it occurs inside positive
assertions, but is ignored in negative assertions.
Simple Assertions
The final use of backslash is for certain simple assertions. An
assertion specifies a condition that must be met at a particular point
in a match, without consuming any characters from the subject string.
The use of subpatterns for more complicated assertions is described
below. The following are the backslashed assertions:
:
Matches at a word boundary.
\B:
Matches when not at a word boundary.
\A:
Matches at the start of the subject.
\Z:
Matches at the end of the subject, and before a newline at the end
of the subject.
\z:
Matches only at the end of the subject.
\G:
Matches at the first matching position in the subject.
Inside a character class, has a different meaning; it matches the
backspace character. If any other of these assertions appears in a
character class, by default it matches the corresponding literal
character (for example, \B matches the letter B).
A word boundary is a position in the subject string where the current
character and the previous character do not both match \w or \W (that
is, one matches \w and the other matches \W), or the start or end of
the string if the first or last character matches \w, respectively. In
UTF mode, the meanings of \w and \W can be changed by setting option
ucp. When this is done, it also affects and \B. PCRE and Perl do not
have a separate "start of word" or "end of word" metasequence. However,
whatever follows normally determines which it is. For example, the
fragment a matches "a" at the start of a word.
The \A, \Z, and \z assertions differ from the traditional circumflex
and dollar (described in the next section) in that they only ever match
at the very start and end of the subject string, whatever options are
set. Thus, they are independent of multiline mode. These three
assertions are not affected by options notbol or noteol, which affect
only the behavior of the circumflex and dollar metacharacters. However,
if argument startoffset of run/3 is non-zero, indicating that matching
is to start at a point other than the beginning of the subject, \A can
never match. The difference between \Z and \z is that \Z matches before
a newline at the end of the string and at the very end, while \z
matches only at the end.
The \G assertion is true only when the current matching position is at
the start point of the match, as specified by argument startoffset of
run/3. It differs from \A when the value of startoffset is non-zero. By
calling run/3 multiple times with appropriate arguments, you can mimic
the Perl option /g, and it is in this kind of implementation where \G
can be useful.
Notice, however, that the PCRE interpretation of \G, as the start of
the current match, is subtly different from Perl, which defines it as
the end of the previous match. In Perl, these can be different when the
previously matched string was empty. As PCRE does only one match at a
time, it cannot reproduce this behavior.
If all the alternatives of a pattern begin with \G, the expression is
anchored to the starting match position, and the "anchored" flag is set
in the compiled regular expression.
CIRCUMFLEX AND DOLLAR
The circumflex and dollar metacharacters are zero-width assertions. That is, they test for a particular condition to be true without consuming any characters from the subject string. Outside a character class, in the default matching mode, the circumflex character is an assertion that is true only if the current matching point is at the start of the subject string. If argument startoffset of run/3 is non-zero, circumflex can never match if option multiline is unset. Inside a character class, circumflex has an entirely different meaning (see below). Circumflex needs not to be the first character of the pattern if some alternatives are involved, but it is to be the first thing in each alternative in which it appears if the pattern is ever to match that branch. If all possible alternatives start with a circumflex, that is, if the pattern is constrained to match only at the start of the subject, it is said to be an "anchored" pattern. (There are also other constructs that can cause a pattern to be anchored.) The dollar character is an assertion that is true only if the current matching point is at the end of the subject string, or immediately before a newline at the end of the string (by default). Notice however that it does not match the newline. Dollar needs not to be the last character of the pattern if some alternatives are involved, but it is to be the last item in any branch in which it appears. Dollar has no special meaning in a character class. The meaning of dollar can be changed so that it matches only at the very end of the string, by setting option dollar_endonly at compile time. This does not affect the \Z assertion. The meanings of the circumflex and dollar characters are changed if option multiline is set. When this is the case, a circumflex matches immediately after internal newlines and at the start of the subject string. It does not match after a newline that ends the string. A dollar matches before any newlines in the string, and at the very end, when multiline is set. When newline is specified as the two-character sequence CRLF, isolated CR and LF characters do not indicate newlines. For example, the pattern /^abc$/ matches the subject string "def\nabc" (where \n represents a newline) in multiline mode, but not otherwise. So, patterns that are anchored in single-line mode because all branches start with ^ are not anchored in multiline mode, and a match for circumflex is possible when argument startoffset of run/3 is non-zero. Option dollar_endonly is ignored if multiline is set. Notice that the sequences \A, \Z, and \z can be used to match the start and end of the subject in both modes. If all branches of a pattern start with \A, it is always anchored, regardless if multiline is set.
FULL STOP (PERIOD, DOT) AND \N
Outside a character class, a dot in the pattern matches any character in the subject string except (by default) a character that signifies the end of a line. When a line ending is defined as a single character, dot never matches that character. When the two-character sequence CRLF is used, dot does not match CR if it is immediately followed by LF, otherwise it matches all characters (including isolated CRs and LFs). When any Unicode line endings are recognized, dot does not match CR, LF, or any of the other line-ending characters. The behavior of dot regarding newlines can be changed. If option dotall is set, a dot matches any character, without exception. If the two- character sequence CRLF is present in the subject string, it takes two dots to match it. The handling of dot is entirely independent of the handling of circumflex and dollar, the only relationship is that both involve newlines. Dot has no special meaning in a character class. The escape sequence \N behaves like a dot, except that it is not affected by option PCRE_DOTALL. That is, it matches any character except one that signifies the end of a line. Perl also uses \N to match characters by name but PCRE does not support this.
MATCHING A SINGLE DATA UNIT
Outside a character class, the escape sequence \C matches any data
unit, regardless if a UTF mode is set. One data unit is one byte.
Unlike a dot, \C always matches line-ending characters. The feature is
provided in Perl to match individual bytes in UTF-8 mode, but it is
unclear how it can usefully be used. As \C breaks up characters into
individual data units, matching one unit with \C in a UTF mode means
that the remaining string can start with a malformed UTF character.
This has undefined results, as PCRE assumes that it deals with valid
UTF strings.
PCRE does not allow \C to appear in lookbehind assertions (described
below) in a UTF mode, as this would make it impossible to calculate the
length of the lookbehind.
The \C escape sequence is best avoided. However, one way of using it
that avoids the problem of malformed UTF characters is to use a
lookahead to check the length of the next character, as in the
following pattern, which can be used with a UTF-8 string (ignore
whitespace and line breaks):
(?| (?=[\x00-\x7f])(\C) |
(?=[\x80-\x{7ff}])(\C)(\C) |
(?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
(?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
A group that starts with (?| resets the capturing parentheses numbers
in each alternative (see section Duplicate Subpattern Numbers). The
assertions at the start of each branch check the next UTF-8 character
for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
individual bytes of the character are then captured by the appropriate
number of groups.
SQUARE BRACKETS AND CHARACTER CLASSES
An opening square bracket introduces a character class, terminated by a
closing square bracket. A closing square bracket on its own is not
special by default. However, if option PCRE_JAVASCRIPT_COMPAT is set, a
lone closing square bracket causes a compile-time error. If a closing
square bracket is required as a member of the class, it is to be the
first data character in the class (after an initial circumflex, if
present) or escaped with a backslash.
A character class matches a single character in the subject. In a UTF
mode, the character can be more than one data unit long. A matched
character must be in the set of characters defined by the class, unless
the first character in the class definition is a circumflex, in which
case the subject character must not be in the set defined by the class.
If a circumflex is required as a member of the class, ensure that it is
not the first character, or escape it with a backslash.
For example, the character class [aeiou] matches any lowercase vowel,
while [^aeiou] matches any character that is not a lowercase vowel.
Notice that a circumflex is just a convenient notation for specifying
the characters that are in the class by enumerating those that are not.
A class that starts with a circumflex is not an assertion; it still
consumes a character from the subject string, and therefore it fails if
the current pointer is at the end of the string.
In UTF-8 mode, characters with values > 255 (0xffff) can be included in
a class as a literal string of data units, or by using the \x{ escaping
mechanism.
When caseless matching is set, any letters in a class represent both
their uppercase and lowercase versions. For example, a caseless [aeiou]
matches "A" and "a", and a caseless [^aeiou] does not match "A", but a
caseful version would. In a UTF mode, PCRE always understands the
concept of case for characters whose values are < 256, so caseless
matching is always possible. For characters with higher values, the
concept of case is supported only if PCRE is compiled with Unicode
property support. If you want to use caseless matching in a UTF mode
for characters >=, ensure that PCRE is compiled with Unicode property
support and with UTF support.
Characters that can indicate line breaks are never treated in any
special way when matching character classes, whatever line-ending
sequence is in use, and whatever setting of options PCRE_DOTALL and
PCRE_MULTILINE is used. A class such as [^a] always matches one of
these characters.
The minus (hyphen) character can be used to specify a range of
characters in a character class. For example, [d-m] matches any letter
between d and m, inclusive. If a minus character is required in a
class, it must be escaped with a backslash or appear in a position
where it cannot be interpreted as indicating a range, typically as the
first or last character in the class.
The literal character "]" cannot be the end character of a range. A
pattern such as [W-]46] is interpreted as a class of two characters
("W" and "-") followed by a literal string "46]", so it would match
"W46]" or "-46]". However, if "]" is escaped with a backslash, it is
interpreted as the end of range, so [W-\]46] is interpreted as a class
containing a range followed by two other characters. The octal or
hexadecimal representation of "]" can also be used to end a range.
Ranges operate in the collating sequence of character values. They can
also be used for characters specified numerically, for example,
[\000-\037]. Ranges can include any characters that are valid for the
current mode.
If a range that includes letters is used when caseless matching is set,
it matches the letters in either case. For example, [W-c] is equivalent
to [][\\^_`wxyzabc], matched caselessly. In a non-UTF mode, if
character tables for a French locale are in use, [\xc8-\xcb] matches
accented E characters in both cases. In UTF modes, PCRE supports the
concept of case for characters with values > 255 only when it is
compiled with Unicode property support.
The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V,
\w, and \W can appear in a character class, and add the characters that
they match to the class. For example, [\dABCDEF] matches any
hexadecimal digit. In UTF modes, option ucp affects the meanings of \d,
\s, \w and their uppercase partners, just as it does when they appear
outside a character class, as described in section Generic Character
Types earlier. The escape sequence has a different meaning inside a
character class; it matches the backspace character. The sequences \B,
\N, \R, and \X are not special inside a character class. Like any other
unrecognized escape sequences, they are treated as the literal
characters "B", "N", "R", and "X".
A circumflex can conveniently be used with the uppercase character
types to specify a more restricted set of characters than the matching
lowercase type. For example, class [^\W_] matches any letter or digit,
but not underscore, while [\w] includes underscore. A positive
character class is to be read as "something OR something OR ..." and a
negative class as "NOT something AND NOT something AND NOT ...".
Only the following metacharacters are recognized in character classes:
* Backslash
* Hyphen (only where it can be interpreted as specifying a range)
* Circumflex (only at the start)
* Opening square bracket (only when it can be interpreted as
introducing a Posix class name; see the next section)
* Terminating closing square bracket
However, escaping other non-alphanumeric characters does no harm.
POSIX CHARACTER CLASSES
Perl supports the Posix notation for character classes. This uses names
enclosed by [: and :] within the enclosing square brackets. PCRE also
supports this notation. For example, the following matches "0", "1",
any alphabetic character, or "%":
[01[:alpha:]%]
The following are the supported class names:
alnum:
Letters and digits
alpha:
Letters
ascii:
Character codes 0-127
blank:
Space or tab only
cntrl:
Control characters
digit:
Decimal digits (same as \d)
graph:
Printing characters, excluding space
lower:
Lowercase letters
print:
Printing characters, including space
punct:
Printing characters, excluding letters, digits, and space
space:
Whitespace (not quite the same as \s)
upper:
Uppercase letters
word:
"Word" characters (same as \w)
xdigit:
Hexadecimal digits
The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
and space (32). Notice that this list includes the VT character (code
11). This makes "space" different to \s, which does not include VT (for
Perl compatibility).
The name "word" is a Perl extension, and "blank" is a GNU extension
from Perl 5.8. Another Perl extension is negation, which is indicated
by a ^ character after the colon. For example, the following matches
"1", "2", or any non-digit:
[12[:^digit:]]
PCRE (and Perl) also recognize the Posix syntax [.ch.] and [=ch=] where
"ch" is a "collating element", but these are not supported, and an
error is given if they are encountered.
By default, in UTF modes, characters with values > 255 do not match any
of the Posix character classes. However, if option PCRE_UCP is passed
to pcre_compile(), some of the classes are changed so that Unicode
character properties are used. This is achieved by replacing the Posix
classes by other sequences, as follows:
[:alnum:]:
Becomes \p{Xan}
[:alpha:]:
Becomes \p{L}
[:blank:]:
Becomes \h
[:digit:]:
Becomes \p{Nd}
[:lower:]:
Becomes \p{Ll}
[:space:]:
Becomes \p{Xps}
[:upper:]:
Becomes \p{Lu}
[:word:]:
Becomes \p{Xwd}
Negated versions, such as [:^alpha:], use \P instead of \p. The other
Posix classes are unchanged, and match only characters with code points
< 256.
VERTICAL BAR
Vertical bar characters are used to separate alternative patterns. For example, the following pattern matches either "gilbert" or "sullivan": gilbert|sullivan Any number of alternatives can appear, and an empty alternative is permitted (matching the empty string). The matching process tries each alternative in turn, from left to right, and the first that succeeds is used. If the alternatives are within a subpattern (defined in section Subpatterns), "succeeds" means matching the remaining main pattern and the alternative in the subpattern.
INTERNAL OPTION SETTING
The settings of the Perl-compatible options caseless, multiline,
dotall, and extended can be changed from within the pattern by a
sequence of Perl option letters enclosed between "(?" and ")". The
option letters are as follows:
i:
For caseless
m:
For multiline
s:
For dotall
x:
For extended
For example, (?im) sets caseless, multiline matching. These options can
also be unset by preceding the letter with a hyphen. A combined setting
and unsetting such as (?im-sx), which sets caseless and multiline,
while unsetting dotall and extended, is also permitted. If a letter
appears both before and after the hyphen, the option is unset.
The PCRE-specific options dupnames, ungreedy, and extra can be changed
in the same way as the Perl-compatible options by using the characters
J, U, and X respectively.
When one of these option changes occurs at top-level (that is, not
inside subpattern parentheses), the change applies to the remainder of
the pattern that follows. If the change is placed right at the start of
a pattern, PCRE extracts it into the global options.
An option change within a subpattern (see section Subpatterns) affects
only that part of the subpattern that follows it. So, the following
matches abc and aBc and no other strings (assuming caseless is not
used):
(a(?i)b)c
By this means, options can be made to have different settings in
different parts of the pattern. Any changes made in one alternative do
carry on into subsequent branches within the same subpattern. For
example:
(a(?i)b|c)
matches "ab", "aB", "c", and "C", although when matching "C" the first
branch is abandoned before the option setting. This is because the
effects of option settings occur at compile time. There would be some
weird behavior otherwise.
Note:
Other PCRE-specific options can be set by the application when the
compiling or matching functions are called. Sometimes the pattern can
contain special leading sequences, such as (*CRLF), to override what
the application has set or what has been defaulted. Details are
provided in section Newline Sequences earlier.
The (*UTF8) and (*UCP) leading sequences can be used to set UTF and
Unicode property modes. They are equivalent to setting options unicode
and ucp, respectively. The (*UTF) sequence is a generic version that
can be used with any of the libraries. However, the application can set
option never_utf, which locks out the use of the (*UTF) sequences.
SUBPATTERNS
Subpatterns are delimited by parentheses (round brackets), which can be
nested. Turning part of a pattern into a subpattern does two things:
1.:
It localizes a set of alternatives. For example, the following
pattern matches "cataract", "caterpillar", or "cat":
cat(aract|erpillar|)
Without the parentheses, it would match "cataract", "erpillar", or
an empty string.
2.:
It sets up the subpattern as a capturing subpattern. That is, when
the complete pattern matches, that portion of the subject string
that matched the subpattern is passed back to the caller through
the return value of run/3.
Opening parentheses are counted from left to right (starting from 1) to
obtain numbers for the capturing subpatterns. For example, if the
string "the red king" is matched against the following pattern, the
captured substrings are "red king", "red", and "king", and are numbered
1, 2, and 3, respectively:
the ((red|white) (king|queen))
It is not always helpful that plain parentheses fulfill two functions.
Often a grouping subpattern is required without a capturing
requirement. If an opening parenthesis is followed by a question mark
and a colon, the subpattern does not do any capturing, and is not
counted when computing the number of any subsequent capturing
subpatterns. For example, if the string "the white queen" is matched
against the following pattern, the captured substrings are "white
queen" and "queen", and are numbered 1 and 2:
the ((?:red|white) (king|queen))
The maximum number of capturing subpatterns is 65535.
As a convenient shorthand, if any option settings are required at the
start of a non-capturing subpattern, the option letters can appear
between "?" and ":". Thus, the following two patterns match the same
set of strings:
(?i:saturday|sunday)
(?:(?i)saturday|sunday)
As alternative branches are tried from left to right, and options are
not reset until the end of the subpattern is reached, an option setting
in one branch does affect subsequent branches, so the above patterns
match both "SUNDAY" and "Saturday".
DUPLICATE SUBPATTERN NUMBERS
Perl 5.10 introduced a feature where each alternative in a subpattern uses the same numbers for its capturing parentheses. Such a subpattern starts with (?| and is itself a non-capturing subpattern. For example, consider the following pattern: (?|(Sat)ur|(Sun))day As the two alternatives are inside a (?| group, both sets of capturing parentheses are numbered one. Thus, when the pattern matches, you can look at captured substring number one, whichever alternative matched. This construct is useful when you want to capture a part, but not all, of one of many alternatives. Inside a (?| group, parentheses are numbered as usual, but the number is reset at the start of each branch. The numbers of any capturing parentheses that follow the subpattern start after the highest number used in any branch. The following example is from the Perl documentation; the numbers underneath show in which buffer the captured content is stored: # before ---------------branch-reset----------- after / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x # 1 2 2 3 2 3 4 A back reference to a numbered subpattern uses the most recent value that is set for that number by any subpattern. The following pattern matches "abcabc" or "defdef": /(?|(abc)|(def))\1/ In contrast, a subroutine call to a numbered subpattern always refers to the first one in the pattern with the given number. The following pattern matches "abcabc" or "defabc": /(?|(abc)|(def))(?1)/ If a condition test for a subpattern having matched refers to a non- unique number, the test is true if any of the subpatterns of that number have matched. An alternative approach using this "branch reset" feature is to use duplicate named subpatterns, as described in the next section.
NAMED SUBPATTERNS
Identifying capturing parentheses by number is simple, but it can be hard to keep track of the numbers in complicated regular expressions. Also, if an expression is modified, the numbers can change. To help with this difficulty, PCRE supports the naming of subpatterns. This feature was not added to Perl until release 5.10. Python had the feature earlier, and PCRE introduced it at release 4.0, using the Python syntax. PCRE now supports both the Perl and the Python syntax. Perl allows identically numbered subpatterns to have different names, but PCRE does not. In PCRE, a subpattern can be named in one of three ways: (?<name>...) or (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing parentheses from other parts of the pattern, such as back references, recursion, and conditions, can be made by name and by number. Names consist of up to 32 alphanumeric characters and underscores. Named capturing parentheses are still allocated numbers as well as names, exactly as if the names were not present. The capture specification to run/3 can use named values if they are present in the regular expression. By default, a name must be unique within a pattern, but this constraint can be relaxed by setting option dupnames at compile time. (Duplicate names are also always permitted for subpatterns with the same number, set up as described in the previous section.) Duplicate names can be useful for patterns where only one instance of the named parentheses can match. Suppose that you want to match the name of a weekday, either as a 3-letter abbreviation or as the full name, and in both cases you want to extract the abbreviation. The following pattern (ignoring the line breaks) does the job: (?<DN>Mon|Fri|Sun)(?:day)?| (?<DN>Tue)(?:sday)?| (?<DN>Wed)(?:nesday)?| (?<DN>Thu)(?:rsday)?| (?<DN>Sat)(?:urday)? There are five capturing substrings, but only one is ever set after a match. (An alternative way of solving this problem is to use a "branch reset" subpattern, as described in the previous section.) For capturing named subpatterns which names are not unique, the first matching occurrence (counted from left to right in the subject) is returned from run/3, if the name is specified in the values part of the capture statement. The all_names capturing value matches all the names in the same way. Note: You cannot use different names to distinguish between two subpatterns with the same number, as PCRE uses only the numbers when matching. For this reason, an error is given at compile time if different names are specified to subpatterns with the same number. However, you can specify the same name to subpatterns with the same number, even when dupnames is not set.
REPETITION
Repetition is specified by quantifiers, which can follow any of the
following items:
* A literal data character
* The dot metacharacter
* The \C escape sequence
* The \X escape sequence
* The \R escape sequence
* An escape such as \d or \pL that matches a single character
* A character class
* A back reference (see the next section)
* A parenthesized subpattern (including assertions)
* A subroutine call to a subpattern (recursive or otherwise)
The general repetition quantifier specifies a minimum and maximum
number of permitted matches, by giving the two numbers in curly
brackets (braces), separated by a comma. The numbers must be < 65536,
and the first must be less than or equal to the second. For example,
the following matches "zz", "zzz", or "zzzz":
z{2,4}
A closing brace on its own is not a special character. If the second
number is omitted, but the comma is present, there is no upper limit.
If the second number and the comma are both omitted, the quantifier
specifies an exact number of required matches. Thus, the following
matches at least three successive vowels, but can match many more:
[aeiou]{3,}
The following matches exactly eight digits:
\d{8}
An opening curly bracket that appears in a position where a quantifier
is not allowed, or one that does not match the syntax of a quantifier,
is taken as a literal character. For example, {,6} is not a quantifier,
but a literal string of four characters.
In Unicode mode, quantifiers apply to characters rather than to
individual data units. Thus, for example, \x{100}{2} matches two
characters, each of which is represented by a 2-byte sequence in a
UTF-8 string. Similarly, \X{3} matches three Unicode extended grapheme
clusters, each of which can be many data units long (and they can be of
different lengths).
The quantifier {0} is permitted, causing the expression to behave as if
the previous item and the quantifier were not present. This can be
useful for subpatterns that are referenced as subroutines from
elsewhere in the pattern (but see also section Defining Subpatterns
for Use by Reference Only). Items other than subpatterns that have a
{0} quantifier are omitted from the compiled pattern.
For convenience, the three most common quantifiers have single-
character abbreviations:
*:
Equivalent to {0,}
+:
Equivalent to {1,}
?:
Equivalent to {0,1}
Infinite loops can be constructed by following a subpattern that can
match no characters with a quantifier that has no upper limit, for
example:
(a?)*
Earlier versions of Perl and PCRE used to give an error at compile time
for such patterns. However, as there are cases where this can be
useful, such patterns are now accepted. However, if any repetition of
the subpattern matches no characters, the loop is forcibly broken.
By default, the quantifiers are "greedy", that is, they match as much
as possible (up to the maximum number of permitted times), without
causing the remaining pattern to fail. The classic example of where
this gives problems is in trying to match comments in C programs. These
appear between /* and */. Within the comment, individual * and /
characters can appear. An attempt to match C comments by applying the
pattern
/\*.*\*/
to the string
/* first comment */ not comment /* second comment */
fails, as it matches the entire string owing to the greediness of the
.* item.
However, if a quantifier is followed by a question mark, it ceases to
be greedy, and instead matches the minimum number of times possible, so
the following pattern does the right thing with the C comments:
/\*.*?\*/
The meaning of the various quantifiers is not otherwise changed, only
the preferred number of matches. Do not confuse this use of question
mark with its use as a quantifier in its own right. As it has two uses,
it can sometimes appear doubled, as in
\d??\d
which matches one digit by preference, but can match two if that is the
only way the remaining pattern matches.
If option ungreedy is set (an option that is not available in Perl),
the quantifiers are not greedy by default, but individual ones can be
made greedy by following them with a question mark. That is, it inverts
the default behavior.
When a parenthesized subpattern is quantified with a minimum repeat
count that is > 1 or with a limited maximum, more memory is required
for the compiled pattern, in proportion to the size of the minimum or
maximum.
If a pattern starts with .* or .{0,} and option dotall (equivalent to
Perl option /s) is set, thus allowing the dot to match newlines, the
pattern is implicitly anchored, because whatever follows is tried
against every character position in the subject string. So, there is no
point in retrying the overall match at any position after the first.
PCRE normally treats such a pattern as if it was preceded by \A.
In cases where it is known that the subject string contains no
newlines, it is worth setting dotall to obtain this optimization, or
alternatively using ^ to indicate anchoring explicitly.
However, there are some cases where the optimization cannot be used.
When .* is inside capturing parentheses that are the subject of a back
reference elsewhere in the pattern, a match at the start can fail where
a later one succeeds. Consider, for example:
(.*)abc\1
If the subject is "xyz123abc123", the match point is the fourth
character. Therefore, such a pattern is not implicitly anchored.
Another case where implicit anchoring is not applied is when the
leading .* is inside an atomic group. Once again, a match at the start
can fail where a later one succeeds. Consider the following pattern:
(?>.*?a)b
It matches "ab" in the subject "aab". The use of the backtracking
control verbs (*PRUNE) and (*SKIP) also disable this optimization.
When a capturing subpattern is repeated, the value captured is the
substring that matched the final iteration. For example, after
(tweedle[dume]{3}\s*)+
has matched "tweedledum tweedledee", the value of the captured
substring is "tweedledee". However, if there are nested capturing
subpatterns, the corresponding captured values can have been set in
previous iterations. For example, after
/(a|(b))+/
matches "aba", the value of the second captured substring is "b".
ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
repetition, failure of what follows normally causes the repeated item
to be re-evaluated to see if a different number of repeats allows the
remaining pattern to match. Sometimes it is useful to prevent this,
either to change the nature of the match, or to cause it to fail
earlier than it otherwise might, when the author of the pattern knows
that there is no point in carrying on.
Consider, for example, the pattern \d+foo when applied to the following
subject line:
123456bar
After matching all six digits and then failing to match "foo", the
normal action of the matcher is to try again with only five digits
matching item \d+, and then with four, and so on, before ultimately
failing. "Atomic grouping" (a term taken from Jeffrey Friedl's book)
provides the means for specifying that once a subpattern has matched,
it is not to be re-evaluated in this way.
If atomic grouping is used for the previous example, the matcher gives
up immediately on failing to match "foo" the first time. The notation
is a kind of special parenthesis, starting with (?> as in the following
example:
(?>\d+)foo
This kind of parenthesis "locks up" the part of the pattern it contains
once it has matched, and a failure further into the pattern is
prevented from backtracking into it. Backtracking past it to previous
items, however, works as normal.
An alternative description is that a subpattern of this type matches
the string of characters that an identical standalone pattern would
match, if anchored at the current point in the subject string.
Atomic grouping subpatterns are not capturing subpatterns. Simple cases
such as the above example can be thought of as a maximizing repeat that
must swallow everything it can. So, while both \d+ and \d+? are
prepared to adjust the number of digits they match to make the
remaining pattern match, (?>\d+) can only match an entire sequence of
digits.
Atomic groups in general can contain any complicated subpatterns, and
can be nested. However, when the subpattern for an atomic group is just
a single repeated item, as in the example above, a simpler notation,
called a "possessive quantifier" can be used. This consists of an extra
+ character following a quantifier. Using this notation, the previous
example can be rewritten as
\d++foo
Notice that a possessive quantifier can be used with an entire group,
for example:
(abc|xyz){2,3}+
Possessive quantifiers are always greedy; the setting of option
ungreedy is ignored. They are a convenient notation for the simpler
forms of an atomic group. However, there is no difference in the
meaning of a possessive quantifier and the equivalent atomic group, but
there can be a performance difference; possessive quantifiers are
probably slightly faster.
The possessive quantifier syntax is an extension to the Perl 5.8
syntax. Jeffrey Friedl originated the idea (and the name) in the first
edition of his book. Mike McCloskey liked it, so implemented it when he
built the Sun Java package, and PCRE copied it from there. It
ultimately found its way into Perl at release 5.10.
PCRE has an optimization that automatically "possessifies" certain
simple pattern constructs. For example, the sequence A+B is treated as
A++B, as there is no point in backtracking into a sequence of A:s when
B must follow.
When a pattern contains an unlimited repeat inside a subpattern that
can itself be repeated an unlimited number of times, the use of an
atomic group is the only way to avoid some failing matches taking a
long time. The pattern
(\D+|<\d+>)*[!?]
matches an unlimited number of substrings that either consist of non-
digits, or digits enclosed in <>, followed by ! or ?. When it matches,
it runs quickly. However, if it is applied to
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
it takes a long time before reporting failure. This is because the
string can be divided between the internal \D+ repeat and the external
* repeat in many ways, and all must be tried. (The example uses [!?]
rather than a single character at the end, as both PCRE and Perl have
an optimization that allows for fast failure when a single character is
used. They remember the last single character that is required for a
match, and fail early if it is not present in the string.) If the
pattern is changed so that it uses an atomic group, like the following,
sequences of non-digits cannot be broken, and failure happens quickly:
((?>\D+)|<\d+>)*[!?]
BACK REFERENCES
Outside a character class, a backslash followed by a digit > 0 (and
possibly further digits) is a back reference to a capturing subpattern
earlier (that is, to its left) in the pattern, provided there have been
that many previous capturing left parentheses.
However, if the decimal number following the backslash is < 10, it is
always taken as a back reference, and causes an error only if there are
not that many capturing left parentheses in the entire pattern. That
is, the parentheses that are referenced do need not be to the left of
the reference for numbers < 10. A "forward back reference" of this type
can make sense when a repetition is involved and the subpattern to the
right has participated in an earlier iteration.
It is not possible to have a numerical "forward back reference" to a
subpattern whose number is 10 or more using this syntax, as a sequence
such as \50 is interpreted as a character defined in octal. For more
details of the handling of digits following a backslash, see section
Non-Printing Characters earlier. There is no such problem when named
parentheses are used. A back reference to any subpattern is possible
using named parentheses (see below).
Another way to avoid the ambiguity inherent in the use of digits
following a backslash is to use the \g escape sequence. This escape
must be followed by an unsigned number or a negative number, optionally
enclosed in braces. The following examples are identical:
(ring), \1
(ring), \g1
(ring), \g{1}
An unsigned number specifies an absolute reference without the
ambiguity that is present in the older syntax. It is also useful when
literal digits follow the reference. A negative number is a relative
reference. Consider the following example:
(abc(def)ghi)\g{-1}
The sequence \g{-1} is a reference to the most recently started
capturing subpattern before \g, that is, it is equivalent to \2 in this
example. Similarly, \g{-2} would be equivalent to \1. The use of
relative references can be helpful in long patterns, and also in
patterns that are created by joining fragments containing references
within themselves.
A back reference matches whatever matched the capturing subpattern in
the current subject string, rather than anything matching the
subpattern itself (section Subpattern as Subroutines describes a way of
doing that). So, the following pattern matches "sense and sensibility"
and "response and responsibility", but not "sense and responsibility":
(sens|respons)e and \1ibility
If caseful matching is in force at the time of the back reference, the
case of letters is relevant. For example, the following matches "rah
rah" and "RAH RAH", but not "RAH rah", although the original capturing
subpattern is matched caselessly:
((?i)rah)\s+\1
There are many different ways of writing back references to named
subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
\k'name' are supported, as is the Python syntax (?P=name). The unified
back reference syntax in Perl 5.10, in which \g can be used for both
numeric and named references, is also supported. The previous example
can be rewritten in the following ways:
(?<p1>(?i)rah)\s+\k<p1>
(?'p1'(?i)rah)\s+\k{p1}
(?P<p1>(?i)rah)\s+(?P=p1)
(?<p1>(?i)rah)\s+\g{p1}
A subpattern that is referenced by name can appear in the pattern
before or after the reference.
There can be more than one back reference to the same subpattern. If a
subpattern has not been used in a particular match, any back references
to it always fails. For example, the following pattern always fails if
it starts to match "a" rather than "bc":
(a|(bc))\2
As there can be many capturing parentheses in a pattern, all digits
following the backslash are taken as part of a potential back reference
number. If the pattern continues with a digit character, some delimiter
must be used to terminate the back reference. If option extended is
set, this can be whitespace. Otherwise an empty comment (see section
Comments) can be used.
Recursive Back References
A back reference that occurs inside the parentheses to which it refers
fails when the subpattern is first used, so, for example, (a\1) never
matches. However, such references can be useful inside repeated
subpatterns. For example, the following pattern matches any number of
"a"s and also "aba", "ababbaa", and so on:
(a|b\1)+
At each iteration of the subpattern, the back reference matches the
character string corresponding to the previous iteration. In order for
this to work, the pattern must be such that the first iteration does
not need to match the back reference. This can be done using
alternation, as in the example above, or by a quantifier with a minimum
of zero.
Back references of this type cause the group that they reference to be
treated as an atomic group. Once the whole group has been matched, a
subsequent matching failure cannot cause backtracking into the middle
of the group.
ASSERTIONS
An assertion is a test on the characters following or preceding the
current matching point that does not consume any characters. The simple
assertions coded as , \B, \A, \G, \Z, \z, ^, and $ are described in
the previous sections.
More complicated assertions are coded as subpatterns. There are two
kinds: those that look ahead of the current position in the subject
string, and those that look behind it. An assertion subpattern is
matched in the normal way, except that it does not cause the current
matching position to be changed.
Assertion subpatterns are not capturing subpatterns. If such an
assertion contains capturing subpatterns within it, these are counted
for the purposes of numbering the capturing subpatterns in the whole
pattern. However, substring capturing is done only for positive
assertions. (Perl sometimes, but not always, performs capturing in
negative assertions.)
For compatibility with Perl, assertion subpatterns can be repeated.
However, it makes no sense to assert the same thing many times, the
side effect of capturing parentheses can occasionally be useful. In
practice, there are only three cases:
* If the quantifier is {0}, the assertion is never obeyed during
matching. However, it can contain internal capturing parenthesized
groups that are called from elsewhere through the subroutine
mechanism.
* If quantifier is {0,n}, where n > 0, it is treated as if it was
{0,1}. At runtime, the remaining pattern match is tried with and
without the assertion, the order depends on the greediness of the
quantifier.
* If the minimum repetition is > 0, the quantifier is ignored. The
assertion is obeyed only once when encountered during matching.
Lookahead Assertions
Lookahead assertions start with (?= for positive assertions and (?! for
negative assertions. For example, the following matches a word followed
by a semicolon, but does not include the semicolon in the match:
\w+(?=;)
The following matches any occurrence of "foo" that is not followed by
"bar":
foo(?!bar)
Notice that the apparently similar pattern
(?!foo)bar
does not find an occurrence of "bar" that is preceded by something
other than "foo". It finds any occurrence of "bar" whatsoever, as the
assertion (?!foo) is always true when the next three characters are
"bar". A lookbehind assertion is needed to achieve the other effect.
If you want to force a matching failure at some point in a pattern, the
most convenient way to do it is with (?!), as an empty string always
matches. So, an assertion that requires there is not to be an empty
string must always fail. The backtracking control verb (*FAIL) or (*F)
is a synonym for (?!).
Lookbehind Assertions
Lookbehind assertions start with (?<= for positive assertions and (?<!
for negative assertions. For example, the following finds an occurrence
of "bar" that is not preceded by "foo":
(?<!foo)bar
The contents of a lookbehind assertion are restricted such that all the
strings it matches must have a fixed length. However, if there are many
top-level alternatives, they do not all have to have the same fixed
length. Thus, the following is permitted:
(?<=bullock|donkey)
The following causes an error at compile time:
(?<!dogs?|cats?)
Branches that match different length strings are permitted only at the
top-level of a lookbehind assertion. This is an extension compared with
Perl, which requires all branches to match the same length of string.
An assertion such as the following is not permitted, as its single top-
level branch can match two different lengths:
(?<=ab(c|de))
However, it is acceptable to PCRE if rewritten to use two top-level
branches:
(?<=abc|abde)
Sometimes the escape sequence \K (see above) can be used instead of a
lookbehind assertion to get round the fixed-length restriction.
The implementation of lookbehind assertions is, for each alternative,
to move the current position back temporarily by the fixed length and
then try to match. If there are insufficient characters before the
current position, the assertion fails.
In a UTF mode, PCRE does not allow the \C escape (which matches a
single data unit even in a UTF mode) to appear in lookbehind
assertions, as it makes it impossible to calculate the length of the
lookbehind. The \X and \R escapes, which can match different numbers of
data units, are not permitted either.
"Subroutine" calls (see below), such as (?2) or (?&X), are permitted in
lookbehinds, as long as the subpattern matches a fixed-length string.
Recursion, however, is not supported.
Possessive quantifiers can be used with lookbehind assertions to
specify efficient matching of fixed-length strings at the end of
subject strings. Consider the following simple pattern when applied to
a long string that does not match:
abcd$
As matching proceeds from left to right, PCRE looks for each "a" in the
subject and then sees if what follows matches the remaining pattern. If
the pattern is specified as
^.*abcd$
the initial .* matches the entire string at first. However, when this
fails (as there is no following "a"), it backtracks to match all but
the last character, then all but the last two characters, and so on.
Once again the search for "a" covers the entire string, from right to
left, so we are no better off. However, if the pattern is written as
^.*+(?<=abcd)
there can be no backtracking for the .*+ item; it can match only the
entire string. The subsequent lookbehind assertion does a single test
on the last four characters. If it fails, the match fails immediately.
For long strings, this approach makes a significant difference to the
processing time.
Using Multiple Assertions
Many assertions (of any sort) can occur in succession. For example, the
following matches "foo" preceded by three digits that are not "999":
(?<=\d{3})(?<!999)foo
Notice that each of the assertions is applied independently at the same
point in the subject string. First there is a check that the previous
three characters are all digits, and then there is a check that the
same three characters are not "999". This pattern does not match "foo"
preceded by six characters, the first of which are digits and the last
three of which are not "999". For example, it does not match
"123abcfoo". A pattern to do that is the following:
(?<=\d{3}...)(?<!999)foo
This time the first assertion looks at the preceding six characters,
checks that the first three are digits, and then the second assertion
checks that the preceding three characters are not "999".
Assertions can be nested in any combination. For example, the following
matches an occurrence of "baz" that is preceded by "bar", which in turn
is not preceded by "foo":
(?<=(?<!foo)bar)baz
The following pattern matches "foo" preceded by three digits and any
three characters that are not "999":
(?<=\d{3}(?!999)...)foo
CONDITIONAL SUBPATTERNS
It is possible to cause the matching process to obey a subpattern
conditionally or to choose between two alternative subpatterns,
depending on the result of an assertion, or whether a specific
capturing subpattern has already been matched. The following are the
two possible forms of conditional subpattern:
(?(condition)yes-pattern)
(?(condition)yes-pattern|no-pattern)
If the condition is satisfied, the yes-pattern is used, otherwise the
no-pattern (if present). If more than two alternatives exist in the
subpattern, a compile-time error occurs. Each of the two alternatives
can itself contain nested subpatterns of any form, including
conditional subpatterns; the restriction to two alternatives applies
only at the level of the condition. The following pattern fragment is
an example where the alternatives are complex:
(?(1) (A|B|C) | (D | (?(2)E|F) | E) )
There are four kinds of condition: references to subpatterns,
references to recursion, a pseudo-condition called DEFINE, and
assertions.
Checking for a Used Subpattern By Number
If the text between the parentheses consists of a sequence of digits,
the condition is true if a capturing subpattern of that number has
previously matched. If more than one capturing subpattern with the same
number exists (see section Duplicate Subpattern Numbers earlier), the
condition is true if any of them have matched. An alternative notation
is to precede the digits with a plus or minus sign. In this case, the
subpattern number is relative rather than absolute. The most recently
opened parentheses can be referenced by (?(-1), the next most recent by
(?(-2), and so on. Inside loops, it can also make sense to refer to
subsequent groups. The next parentheses to be opened can be referenced
as (?(+1), and so on. (The value zero in any of these forms is not
used; it provokes a compile-time error.)
Consider the following pattern, which contains non-significant
whitespace to make it more readable (assume option extended) and to
divide it into three parts for ease of discussion:
( \( )? [^()]+ (?(1) \) )
The first part matches an optional opening parenthesis, and if that
character is present, sets it as the first captured substring. The
second part matches one or more characters that are not parentheses.
The third part is a conditional subpattern that tests whether the first
set of parentheses matched or not. If they did, that is, if subject
started with an opening parenthesis, the condition is true, and so the
yes-pattern is executed and a closing parenthesis is required.
Otherwise, as no-pattern is not present, the subpattern matches
nothing. That is, this pattern matches a sequence of non-parentheses,
optionally enclosed in parentheses.
If this pattern is embedded in a larger one, a relative reference can
be used:
...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
This makes the fragment independent of the parentheses in the larger
pattern.
Checking for a Used Subpattern By Name
Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
used subpattern by name. For compatibility with earlier versions of
PCRE, which had this facility before Perl, the syntax (?(name)...) is
also recognized. However, there is a possible ambiguity with this
syntax, as subpattern names can consist entirely of digits. PCRE looks
first for a named subpattern; if it cannot find one and the name
consists entirely of digits, PCRE looks for a subpattern of that
number, which must be > 0. Using subpattern names that consist entirely
of digits is not recommended.
Rewriting the previous example to use a named subpattern gives:
(?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
If the name used in a condition of this kind is a duplicate, the test
is applied to all subpatterns of the same name, and is true if any one
of them has matched.
Checking for Pattern Recursion
If the condition is the string (R), and there is no subpattern with the
name R, the condition is true if a recursive call to the whole pattern
or any subpattern has been made. If digits or a name preceded by
ampersand follow the letter R, for example:
(?(R3)...) or (?(R&name)...)
the condition is true if the most recent recursion is into a subpattern
whose number or name is given. This condition does not check the entire
recursion stack. If the name used in a condition of this kind is a
duplicate, the test is applied to all subpatterns of the same name, and
is true if any one of them is the most recent recursion.
At "top-level", all these recursion test conditions are false. The
syntax for recursive patterns is described below.
Defining Subpatterns for Use By Reference Only
If the condition is the string (DEFINE), and there is no subpattern
with the name DEFINE, the condition is always false. In this case,
there can be only one alternative in the subpattern. It is always
skipped if control reaches this point in the pattern. The idea of
DEFINE is that it can be used to define "subroutines" that can be
referenced from elsewhere. (The use of subroutines is described below.)
For example, a pattern to match an IPv4 address, such as
"192.168.23.245", can be written like this (ignore whitespace and line
breaks):
(?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) (?&byte) (\.(?&byte)){3}
The first part of the pattern is a DEFINE group inside which is a
another group named "byte" is defined. This matches an individual
component of an IPv4 address (a number < 256). When matching takes
place, this part of the pattern is skipped, as DEFINE acts like a false
condition. The remaining pattern uses references to the named group to
match the four dot-separated components of an IPv4 address, insisting
on a word boundary at each end.
Assertion Conditions
If the condition is not in any of the above formats, it must be an
assertion. This can be a positive or negative lookahead or lookbehind
assertion. Consider the following pattern, containing non-significant
whitespace, and with the two alternatives on the second line:
(?(?=[^a-z]*[a-z])
\d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
The condition is a positive lookahead assertion that matches an
optional sequence of non-letters followed by a letter. That is, it
tests for the presence of at least one letter in the subject. If a
letter is found, the subject is matched against the first alternative,
otherwise it is matched against the second. This pattern matches
strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
letters and dd are digits.
COMMENTS
There are two ways to include comments in patterns that are processed by PCRE. In both cases, the start of the comment must not be in a character class, or in the middle of any other sequence of related characters such as (?: or a subpattern name or number. The characters that make up a comment play no part in the pattern matching. The sequence (?# marks the start of a comment that continues up to the next closing parenthesis. Nested parentheses are not permitted. If option PCRE_EXTENDED is set, an unescaped # character also introduces a comment, which in this case continues to immediately after the next newline character or character sequence in the pattern. Which characters are interpreted as newlines is controlled by the options passed to a compiling function or by a special sequence at the start of the pattern, as described in section Newline Conventions earlier. Notice that the end of this type of comment is a literal newline sequence in the pattern; escape sequences that happen to represent a newline do not count. For example, consider the following pattern when extended is set, and the default newline convention is in force: abc #comment \n still comment On encountering character #, pcre_compile() skips along, looking for a newline in the pattern. The sequence \n is still literal at this stage, so it does not terminate the comment. Only a character with code value 0x0a (the default newline) does so.
RECURSIVE PATTERNS
Consider the problem of matching a string in parentheses, allowing for
unlimited nested parentheses. Without the use of recursion, the best
that can be done is to use a pattern that matches up to some fixed
depth of nesting. It is not possible to handle an arbitrary nesting
depth.
For some time, Perl has provided a facility that allows regular
expressions to recurse (among other things). It does this by
interpolating Perl code in the expression at runtime, and the code can
refer to the expression itself. A Perl pattern using code interpolation
to solve the parentheses problem can be created like this:
$re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
Item (?p{...}) interpolates Perl code at runtime, and in this case
refers recursively to the pattern in which it appears.
Obviously, PCRE cannot support the interpolation of Perl code. Instead,
it supports special syntax for recursion of the entire pattern, and for
individual subpattern recursion. After its introduction in PCRE and
Python, this kind of recursion was later introduced into Perl at
release 5.10.
A special item that consists of (? followed by a number > 0 and a
closing parenthesis is a recursive subroutine call of the subpattern of
the given number, if it occurs inside that subpattern. (If not, it is a
non-recursive subroutine call, which is described in the next section.)
The special item (?R) or (?0) is a recursive call of the entire regular
expression.
This PCRE pattern solves the nested parentheses problem (assume that
option extended is set so that whitespace is ignored):
\( ( [^()]++ | (?R) )* \)
First it matches an opening parenthesis. Then it matches any number of
substrings, which can either be a sequence of non-parentheses or a
recursive match of the pattern itself (that is, a correctly
parenthesized substring). Finally there is a closing parenthesis.
Notice the use of a possessive quantifier to avoid backtracking into
sequences of non-parentheses.
If this was part of a larger pattern, you would not want to recurse the
entire pattern, so instead you can use:
( \( ( [^()]++ | (?1) )* \) )
The pattern is here within parentheses so that the recursion refers to
them instead of the whole pattern.
In a larger pattern, keeping track of parenthesis numbers can be
tricky. This is made easier by the use of relative references. Instead
of (?1) in the pattern above, you can write (?-2) to refer to the
second most recently opened parentheses preceding the recursion. That
is, a negative number counts capturing parentheses leftwards from the
point at which it is encountered.
It is also possible to refer to later opened parentheses, by writing
references such as (?+2). However, these cannot be recursive, as the
reference is not inside the parentheses that are referenced. They are
always non-recursive subroutine calls, as described in the next
section.
An alternative approach is to use named parentheses instead. The Perl
syntax for this is (?&name). The earlier PCRE syntax (?P>name) is also
supported. We can rewrite the above example as follows:
(?<pn> \( ( [^()]++ | (?&pn) )* \) )
If there is more than one subpattern with the same name, the earliest
one is used.
This particular example pattern that we have studied contains nested
unlimited repeats, and so the use of a possessive quantifier for
matching strings of non-parentheses is important when applying the
pattern to strings that do not match. For example, when this pattern is
applied to
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
it gives "no match" quickly. However, if a possessive quantifier is not
used, the match runs for a long time, as there are so many different
ways the + and * repeats can carve up the subject, and all must be
tested before failure can be reported.
At the end of a match, the values of capturing parentheses are those
from the outermost level. If the pattern above is matched against
(ab(cd)ef)
the value for the inner capturing parentheses (numbered 2) is "ef",
which is the last value taken on at the top-level. If a capturing
subpattern is not matched at the top level, its final captured value is
unset, even if it was (temporarily) set at a deeper level during the
matching process.
Do not confuse item (?R) with condition (R), which tests for recursion.
Consider the following pattern, which matches text in angle brackets,
allowing for arbitrary nesting. Only digits are allowed in nested
brackets (that is, when recursing), while any characters are permitted
at the outer level.
< (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
Here (?(R) is the start of a conditional subpattern, with two different
alternatives for the recursive and non-recursive cases. Item (?R) is
the actual recursive call.
Differences in Recursion Processing between PCRE and Perl
Recursion processing in PCRE differs from Perl in two important ways.
In PCRE (like Python, but unlike Perl), a recursive subpattern call is
always treated as an atomic group. That is, once it has matched some of
the subject string, it is never re-entered, even if it contains untried
alternatives and there is a subsequent matching failure. This can be
illustrated by the following pattern, which means to match a
palindromic string containing an odd number of characters (for example,
"a", "aba", "abcba", "abcdcba"):
^(.|(.)(?1)\2)$
The idea is that it either matches a single character, or two identical
characters surrounding a subpalindrome. In Perl, this pattern works; in
PCRE it does not work if the pattern is longer than three characters.
Consider the subject string "abcba".
At the top level, the first character is matched, but as it is not at
the end of the string, the first alternative fails, the second
alternative is taken, and the recursion kicks in. The recursive call to
subpattern 1 successfully matches the next character ("b"). (Notice
that the beginning and end of line tests are not part of the
recursion.)
Back at the top level, the next character ("c") is compared with what
subpattern 2 matched, which was "a". This fails. As the recursion is
treated as an atomic group, there are now no backtracking points, and
so the entire match fails. (Perl can now re-enter the recursion and try
the second alternative.) However, if the pattern is written with the
alternatives in the other order, things are different:
^((.)(?1)\2|.)$
This time, the recursing alternative is tried first, and continues to
recurse until it runs out of characters, at which point the recursion
fails. But this time we have another alternative to try at the higher
level. That is the significant difference: in the previous case the
remaining alternative is at a deeper recursion level, which PCRE cannot
use.
To change the pattern so that it matches all palindromic strings, not
only those with an odd number of characters, it is tempting to change
the pattern to this:
^((.)(?1)\2|.?)$
Again, this works in Perl, but not in PCRE, and for the same reason.
When a deeper recursion has matched a single character, it cannot be
entered again to match an empty string. The solution is to separate the
two cases, and write out the odd and even cases as alternatives at the
higher level:
^(?:((.)(?1)\2|)|((.)(?3)\4|.))
If you want to match typical palindromic phrases, the pattern must
ignore all non-word characters, which can be done as follows:
^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
If run with option caseless, this pattern matches phrases such as "A
man, a plan, a canal: Panama!" and it works well in both PCRE and Perl.
Notice the use of the possessive quantifier *+ to avoid backtracking
into sequences of non-word characters. Without this, PCRE takes much
longer (10 times or more) to match typical phrases, and Perl takes so
long that you think it has gone into a loop.
Note:
The palindrome-matching patterns above work only if the subject string
does not start with a palindrome that is shorter than the entire
string. For example, although "abcba" is correctly matched, if the
subject is "ababa", PCRE finds palindrome "aba" at the start, and then
fails at top level, as the end of the string does not follow. Once
again, it cannot jump back into the recursion to try other
alternatives, so the entire match fails.
The second way in which PCRE and Perl differ in their recursion
processing is in the handling of captured values. In Perl, when a
subpattern is called recursively or as a subpattern (see the next
section), it has no access to any values that were captured outside the
recursion. In PCRE these values can be referenced. Consider the
following pattern:
^(.)(\1|a(?2))
In PCRE, it matches "bab". The first capturing parentheses match "b",
then in the second group, when the back reference \1 fails to match
"b", the second alternative matches "a", and then recurses. In the
recursion, \1 does now match "b" and so the whole match succeeds. In
Perl, the pattern fails to match because inside the recursive call \1
cannot access the externally set value.
SUBPATTERNS AS SUBROUTINES
If the syntax for a recursive subpattern call (either by number or by name) is used outside the parentheses to which it refers, it operates like a subroutine in a programming language. The called subpattern can be defined before or after the reference. A numbered reference can be absolute or relative, as in the following examples: (...(absolute)...)...(?2)... (...(relative)...)...(?-1)... (...(?+1)...(relative)... An earlier example pointed out that the following pattern matches "sense and sensibility" and "response and responsibility", but not "sense and responsibility": (sens|respons)e and \1ibility If instead the following pattern is used, it matches "sense and responsibility" and the other two strings: (sens|respons)e and (?1)ibility Another example is provided in the discussion of DEFINE earlier. All subroutine calls, recursive or not, are always treated as atomic groups. That is, once a subroutine has matched some of the subject string, it is never re-entered, even if it contains untried alternatives and there is a subsequent matching failure. Any capturing parentheses that are set during the subroutine call revert to their previous values afterwards. Processing options such as case-independence are fixed when a subpattern is defined, so if it is used as a subroutine, such options cannot be changed for different calls. For example, the following pattern matches "abcabc" but not "abcABC", as the change of processing option does not affect the called subpattern: (abc)(?i:(?-1))
ONIGURUMA SUBROUTINE SYNTAX
For compatibility with Oniguruma, the non-Perl syntax \g followed by a
name or a number enclosed either in angle brackets or single quotes, is
alternative syntax for referencing a subpattern as a subroutine,
possibly recursively. Here follows two of the examples used above,
rewritten using this syntax:
(?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
(sens|respons)e and \g'1'ibility
PCRE supports an extension to Oniguruma: if a number is preceded by a
plus or minus sign, it is taken as a relative reference, for example:
(abc)(?i:\g<-1>)
Notice that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are
not synonymous. The former is a back reference; the latter is a
subroutine call.
BACKTRACKING CONTROL
Perl 5.10 introduced some "Special Backtracking Control Verbs", which
are still described in the Perl documentation as "experimental and
subject to change or removal in a future version of Perl". It goes on
to say: "Their usage in production code should be noted to avoid
problems during upgrades." The same remarks apply to the PCRE features
described in this section.
The new verbs make use of what was previously invalid syntax: an
opening parenthesis followed by an asterisk. They are generally of the
form (*VERB) or (*VERB:NAME). Some can take either form, possibly
behaving differently depending on whether a name is present. A name is
any sequence of characters that does not include a closing parenthesis.
The maximum name length is 255 in the 8-bit library and 65535 in the
16-bit and 32-bit libraries. If the name is empty, that is, if the
closing parenthesis immediately follows the colon, the effect is as if
the colon was not there. Any number of these verbs can occur in a
pattern.
The behavior of these verbs in repeated groups, assertions, and in
subpatterns called as subroutines (whether or not recursively) is
described below.
Optimizations That Affect Backtracking Verbs
PCRE contains some optimizations that are used to speed up matching by
running some checks at the start of each match attempt. For example, it
can know the minimum length of matching subject, or that a particular
character must be present. When one of these optimizations bypasses the
running of a match, any included backtracking verbs are not processed.
processed. You can suppress the start-of-match optimizations by setting
option no_start_optimize when calling compile/2 or run/3, or by
starting the pattern with (*NO_START_OPT).
Experiments with Perl suggest that it too has similar optimizations,
sometimes leading to anomalous results.
Verbs That Act Immediately
The following verbs act as soon as they are encountered. They must not
be followed by a name.
(*ACCEPT)
This verb causes the match to end successfully, skipping the remainder
of the pattern. However, when it is inside a subpattern that is called
as a subroutine, only that subpattern is ended successfully. Matching
then continues at the outer level. If (*ACCEPT) is triggered in a
positive assertion, the assertion succeeds; in a negative assertion,
the assertion fails.
If (*ACCEPT) is inside capturing parentheses, the data so far is
captured. For example, the following matches "AB", "AAD", or "ACD".
When it matches "AB", "B" is captured by the outer parentheses.
A((?:A|B(*ACCEPT)|C)D)
The following verb causes a matching failure, forcing backtracking to
occur. It is equivalent to (?!) but easier to read.
(*FAIL) or (*F)
The Perl documentation states that it is probably useful only when
combined with (?{}) or (??{}). Those are Perl features that are not
present in PCRE.
A match with the string "aaaa" always fails, but the callout is taken
before each backtrack occurs (in this example, 10 times).
Recording Which Path Was Taken
The main purpose of this verb is to track how a match was arrived at,
although it also has a secondary use in with advancing the match
starting point (see (*SKIP) below).
Note:
In Erlang, there is no interface to retrieve a mark with run/2,3, so
only the secondary purpose is relevant to the Erlang programmer.
The rest of this section is therefore deliberately not adapted for
reading by the Erlang programmer, but the examples can help in
understanding NAMES as they can be used by (*SKIP).
(*MARK:NAME) or (*:NAME)
A name is always required with this verb. There can be as many
instances of (*MARK) as you like in a pattern, and their names do not
have to be unique.
When a match succeeds, the name of the last encountered (*MARK:NAME),
(*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to
the caller as described in section "Extra data for pcre_exec()" in the
pcreapi documentation. In the following example of pcretest output, the
/K modifier requests the retrieval and outputting of (*MARK) data:
re> /X(*MARK:A)Y|X(*MARK:B)Z/K
data> XY
0: XY
MK: A
XZ
0: XZ
MK: B
The (*MARK) name is tagged with "MK:" in this output, and in this
example it indicates which of the two alternatives matched. This is a
more efficient way of obtaining this information than putting each
alternative in its own capturing parentheses.
If a verb with a name is encountered in a positive assertion that is
true, the name is recorded and passed back if it is the last
encountered. This does not occur for negative assertions or failing
positive assertions.
After a partial match or a failed match, the last encountered name in
the entire match process is returned, for example:
re> /X(*MARK:A)Y|X(*MARK:B)Z/K
data> XP
No match, mark = B
Notice that in this unanchored example, the mark is retained from the
match attempt that started at letter "X" in the subject. Subsequent
match attempts starting at "P" and then with an empty string do not get
as far as the (*MARK) item, nevertheless do not reset it.
Verbs That Act after Backtracking
The following verbs do nothing when they are encountered. Matching
continues with what follows, but if there is no subsequent match,
causing a backtrack to the verb, a failure is forced. That is,
backtracking cannot pass to the left of the verb. However, when one of
these verbs appears inside an atomic group or an assertion that is
true, its effect is confined to that group, as once the group has been
matched, there is never any backtracking into it. In this situation,
backtracking can "jump back" to the left of the entire atomic group or
assertion. (Remember also, as stated above, that this localization also
applies in subroutine calls.)
These verbs differ in exactly what kind of failure occurs when
backtracking reaches them. The behavior described below is what occurs
when the verb is not in a subroutine or an assertion. Subsequent
sections cover these special cases.
The following verb, which must not be followed by a name, causes the
whole match to fail outright if there is a later matching failure that
causes backtracking to reach it. Even if the pattern is unanchored, no
further attempts to find a match by advancing the starting point take
place.
(*COMMIT)
If (*COMMIT) is the only backtracking verb that is encountered, once it
has been passed, run/2,3 is committed to find a match at the current
starting point, or not at all, for example:
a+(*COMMIT)b
This matches "xxaab" but not "aacaab". It can be thought of as a kind
of dynamic anchor, or "I've started, so I must finish". The name of the
most recently passed (*MARK) in the path is passed back when (*COMMIT)
forces a match failure.
If more than one backtracking verb exists in a pattern, a different one
that follows (*COMMIT) can be triggered first, so merely passing
(*COMMIT) during a match does not always guarantee that a match must be
at this starting point.
Notice that (*COMMIT) at the start of a pattern is not the same as an
anchor, unless the PCRE start-of-match optimizations are turned off, as
shown in the following example:
1> re:run("xyzabc","(*COMMIT)abc",[{capture,all,list}]).
{match,["abc"]}
2> re:run("xyzabc","(*COMMIT)abc",[{capture,all,list},no_start_optimize]).
nomatch
PCRE knows that any match must start with "a", so the optimization
skips along the subject to "a" before running the first match attempt,
which succeeds. When the optimization is disabled by option
no_start_optimize, the match starts at "x" and so the (*COMMIT) causes
it to fail without trying any other starting points.
The following verb causes the match to fail at the current starting
position in the subject if there is a later matching failure that
causes backtracking to reach it:
(*PRUNE) or (*PRUNE:NAME)
If the pattern is unanchored, the normal "bumpalong" advance to the
next starting character then occurs. Backtracking can occur as usual to
the left of (*PRUNE), before it is reached, or when matching to the
right of (*PRUNE), but if there is no match to the right, backtracking
cannot cross (*PRUNE). In simple cases, the use of (*PRUNE) is just an
alternative to an atomic group or possessive quantifier, but there are
some uses of (*PRUNE) that cannot be expressed in any other way. In an
anchored pattern, (*PRUNE) has the same effect as (*COMMIT).
The behavior of (*PRUNE:NAME) is the not the same as
(*MARK:NAME)(*PRUNE). It is like (*MARK:NAME) in that the name is
remembered for passing back to the caller. However, (*SKIP:NAME)
searches only for names set with (*MARK).
Note:
The fact that (*PRUNE:NAME) remembers the name is useless to the Erlang
programmer, as names cannot be retrieved.
The following verb, when specified without a name, is like (*PRUNE),
except that if the pattern is unanchored, the "bumpalong" advance is
not to the next character, but to the position in the subject where
(*SKIP) was encountered.
(*SKIP)
(*SKIP) signifies that whatever text was matched leading up to it
cannot be part of a successful match. Consider:
a+(*SKIP)b
If the subject is "aaaac...", after the first match attempt fails
(starting at the first character in the string), the starting point
skips on to start the next attempt at "c". Notice that a possessive
quantifier does not have the same effect as this example; although it
would suppress backtracking during the first match attempt, the second
attempt would start at the second character instead of skipping on to
"c".
When (*SKIP) has an associated name, its behavior is modified:
(*SKIP:NAME)
When this is triggered, the previous path through the pattern is
searched for the most recent (*MARK) that has the same name. If one is
found, the "bumpalong" advance is to the subject position that
corresponds to that (*MARK) instead of to where (*SKIP) was
encountered. If no (*MARK) with a matching name is found, (*SKIP) is
ignored.
Notice that (*SKIP:NAME) searches only for names set by (*MARK:NAME).
It ignores names that are set by (*PRUNE:NAME) or (*THEN:NAME).
The following verb causes a skip to the next innermost alternative when
backtracking reaches it. That is, it cancels any further backtracking
within the current alternative.
(*THEN) or (*THEN:NAME)
The verb name comes from the observation that it can be used for a
pattern-based if-then-else block:
( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
If the COND1 pattern matches, FOO is tried (and possibly further items
after the end of the group if FOO succeeds). On failure, the matcher
skips to the second alternative and tries COND2, without backtracking
into COND1. If that succeeds and BAR fails, COND3 is tried. If BAZ then
fails, there are no more alternatives, so there is a backtrack to
whatever came before the entire group. If (*THEN) is not inside an
alternation, it acts like (*PRUNE).
The behavior of (*THEN:NAME) is the not the same as
(*MARK:NAME)(*THEN). It is like (*MARK:NAME) in that the name is
remembered for passing back to the caller. However, (*SKIP:NAME)
searches only for names set with (*MARK).
Note:
The fact that (*THEN:NAME) remembers the name is useless to the Erlang
programmer, as names cannot be retrieved.
A subpattern that does not contain a | character is just a part of the
enclosing alternative; it is not a nested alternation with only one
alternative. The effect of (*THEN) extends beyond such a subpattern to
the enclosing alternative. Consider the following pattern, where A, B,
and so on, are complex pattern fragments that do not contain any |
characters at this level:
A (B(*THEN)C) | D
If A and B are matched, but there is a failure in C, matching does not
backtrack into A; instead it moves to the next alternative, that is, D.
However, if the subpattern containing (*THEN) is given an alternative,
it behaves differently:
A (B(*THEN)C | (*FAIL)) | D
The effect of (*THEN) is now confined to the inner subpattern. After a
failure in C, matching moves to (*FAIL), which causes the whole
subpattern to fail, as there are no more alternatives to try. In this
case, matching does now backtrack into A.
Notice that a conditional subpattern is not considered as having two
alternatives, as only one is ever used. That is, the | character in a
conditional subpattern has a different meaning. Ignoring whitespace,
consider:
^.*? (?(?=a) a | b(*THEN)c )
If the subject is "ba", this pattern does not match. As .*? is
ungreedy, it initially matches zero characters. The condition (?=a)
then fails, the character "b" is matched, but "c" is not. At this
point, matching does not backtrack to .*? as can perhaps be expected
from the presence of the | character. The conditional subpattern is
part of the single alternative that comprises the whole pattern, and so
the match fails. (If there was a backtrack into .*?, allowing it to
match "b", the match would succeed.)
The verbs described above provide four different "strengths" of control
when subsequent matching fails:
* (*THEN) is the weakest, carrying on the match at the next
alternative.
* (*PRUNE) comes next, fails the match at the current starting
position, but allows an advance to the next character (for an
unanchored pattern).
* (*SKIP) is similar, except that the advance can be more than one
character.
* (*COMMIT) is the strongest, causing the entire match to fail.
More than One Backtracking Verb
If more than one backtracking verb is present in a pattern, the one
that is backtracked onto first acts. For example, consider the
following pattern, where A, B, and so on, are complex pattern
fragments:
(A(*COMMIT)B(*THEN)C|ABD)
If A matches but B fails, the backtrack to (*COMMIT) causes the entire
match to fail. However, if A and B match, but C fails, the backtrack to
(*THEN) causes the next alternative (ABD) to be tried. This behavior is
consistent, but is not always the same as in Perl. It means that if two
or more backtracking verbs appear in succession, the last of them has
no effect. Consider the following example:
...(*COMMIT)(*PRUNE)...
If there is a matching failure to the right, backtracking onto (*PRUNE)
cases it to be triggered, and its action is taken. There can never be a
backtrack onto (*COMMIT).
Backtracking Verbs in Repeated Groups
PCRE differs from Perl in its handling of backtracking verbs in
repeated groups. For example, consider:
/(a(*COMMIT)b)+ac/
If the subject is "abac", Perl matches, but PCRE fails because the
(*COMMIT) in the second repeat of the group acts.
Backtracking Verbs in Assertions
(*FAIL) in an assertion has its normal effect: it forces an immediate
backtrack.
(*ACCEPT) in a positive assertion causes the assertion to succeed
without any further processing. In a negative assertion, (*ACCEPT)
causes the assertion to fail without any further processing.
The other backtracking verbs are not treated specially if they appear
in a positive assertion. In particular, (*THEN) skips to the next
alternative in the innermost enclosing group that has alternations,
regardless if this is within the assertion.
Negative assertions are, however, different, to ensure that changing a
positive assertion into a negative assertion changes its result.
Backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative
assertion to be true, without considering any further alternative
branches in the assertion. Backtracking into (*THEN) causes it to skip
to the next enclosing alternative within the assertion (the normal
behavior), but if the assertion does not have such an alternative,
(*THEN) behaves like (*PRUNE).
Backtracking Verbs in Subroutines
These behaviors occur regardless if the subpattern is called
recursively. The treatment of subroutines in Perl is different in some
cases.
* (*FAIL) in a subpattern called as a subroutine has its normal
effect: it forces an immediate backtrack.
* (*ACCEPT) in a subpattern called as a subroutine causes the
subroutine match to succeed without any further processing.
Matching then continues after the subroutine call.
* (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a
subroutine cause the subroutine match to fail.
* (*THEN) skips to the next alternative in the innermost enclosing
group within the subpattern that has alternatives. If there is no
such group within the subpattern, (*THEN) causes the subroutine
match to fail.
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