unicode(3erl)
NAME
unicode - Functions for converting Unicode characters.
DESCRIPTION
This module contains functions for converting between different character representations. It converts between ISO Latin-1 characters and Unicode characters, but it can also convert between different Unicode encodings (like UTF-8, UTF-16, and UTF-32). The default Unicode encoding in Erlang is in binaries UTF-8, which is also the format in which built-in functions and libraries in OTP expect to find binary Unicode data. In lists, Unicode data is encoded as integers, each integer representing one character and encoded simply as the Unicode code point for the character. Other Unicode encodings than integers representing code points or UTF-8 in binaries are referred to as "external encodings". The ISO Latin-1 encoding is in binaries and lists referred to as latin1-encoding. It is recommended to only use external encodings for communication with external entities where this is required. When working inside the Erlang/OTP environment, it is recommended to keep binaries in UTF-8 when representing Unicode characters. ISO Latin-1 encoding is supported both for backward compatibility and for communication with external entities not supporting Unicode character sets.
DATA TYPES
encoding() =
latin1 |
unicode |
utf8 |
utf16 |
{utf16, endian()} |
utf32 |
{utf32, endian()}
endian() = big | little
unicode_binary() = binary()
A binary() with characters encoded in the UTF-8 coding standard.
chardata() = charlist() | unicode_binary()
charlist() =
maybe_improper_list(char() | unicode_binary() | charlist(),
unicode_binary() | [])
external_unicode_binary() = binary()
A binary() with characters coded in a user-specified Unicode
encoding other than UTF-8 (that is, UTF-16 or UTF-32).
external_chardata() =
external_charlist() | external_unicode_binary()
external_charlist() =
maybe_improper_list(char() |
external_unicode_binary() |
external_charlist(),
external_unicode_binary() | [])
latin1_binary() = binary()
A binary() with characters coded in ISO Latin-1.
latin1_char() = byte()
An integer() representing a valid ISO Latin-1 character (0-255).
latin1_chardata() = latin1_charlist() | latin1_binary()
Same as iodata().
latin1_charlist() =
maybe_improper_list(latin1_char() |
latin1_binary() |
latin1_charlist(),
latin1_binary() | [])
Same as iolist().
EXPORTS
bom_to_encoding(Bin) -> {Encoding, Length}
Types:
Bin = binary()
A binary() such that byte_size(Bin) >= 4.
Encoding =
latin1 | utf8 | {utf16, endian()} | {utf32, endian()}
Length = integer() >= 0
endian() = big | little
Checks for a UTF Byte Order Mark (BOM) in the beginning of a
binary. If the supplied binary Bin begins with a valid BOM for
either UTF-8, UTF-16, or UTF-32, the function returns the
encoding identified along with the BOM length in bytes.
If no BOM is found, the function returns {latin1,0}.
characters_to_binary(Data) -> Result
Types:
Data = latin1_chardata() | chardata() | external_chardata()
Result =
binary() |
{error, binary(), RestData} |
{incomplete, binary(), binary()}
RestData = latin1_chardata() | chardata() |
external_chardata()
Same as characters_to_binary(Data, unicode, unicode).
characters_to_binary(Data, InEncoding) -> Result
Types:
Data = latin1_chardata() | chardata() | external_chardata()
InEncoding = encoding()
Result =
binary() |
{error, binary(), RestData} |
{incomplete, binary(), binary()}
RestData = latin1_chardata() | chardata() |
external_chardata()
Same as characters_to_binary(Data, InEncoding, unicode).
characters_to_binary(Data, InEncoding, OutEncoding) -> Result
Types:
Data = latin1_chardata() | chardata() | external_chardata()
InEncoding = OutEncoding = encoding()
Result =
binary() |
{error, binary(), RestData} |
{incomplete, binary(), binary()}
RestData = latin1_chardata() | chardata() |
external_chardata()
Behaves as characters_to_list/2, but produces a binary instead
of a Unicode list.
InEncoding defines how input is to be interpreted if binaries
are present in Data
OutEncoding defines in what format output is to be generated.
Options:
unicode:
An alias for utf8, as this is the preferred encoding for
Unicode characters in binaries.
utf16:
An alias for {utf16,big}.
utf32:
An alias for {utf32,big}.
The atoms big and little denote big- or little-endian encoding.
Errors and exceptions occur as in characters_to_list/2, but the
second element in tuple error or incomplete is a binary() and
not a list().
characters_to_list(Data) -> Result
Types:
Data = latin1_chardata() | chardata() | external_chardata()
Result =
list() |
{error, list(), RestData} |
{incomplete, list(), binary()}
RestData = latin1_chardata() | chardata() |
external_chardata()
Same as characters_to_list(Data, unicode).
characters_to_list(Data, InEncoding) -> Result
Types:
Data = latin1_chardata() | chardata() | external_chardata()
InEncoding = encoding()
Result =
list() |
{error, list(), RestData} |
{incomplete, list(), binary()}
RestData = latin1_chardata() | chardata() |
external_chardata()
Converts a possibly deep list of integers and binaries into a
list of integers representing Unicode characters. The binaries
in the input can have characters encoded as one of the
following:
* ISO Latin-1 (0-255, one character per byte). Here, case
parameter InEncoding is to be specified as latin1.
* One of the UTF-encodings, which is specified as parameter
InEncoding.
Only when InEncoding is one of the UTF encodings, integers in
the list are allowed to be > 255.
If InEncoding is latin1, parameter Data corresponds to the
iodata() type, but for unicode, parameter Data can contain
integers > 255 (Unicode characters beyond the ISO Latin-1
range), which makes it invalid as iodata().
The purpose of the function is mainly to convert combinations of
Unicode characters into a pure Unicode string in list
representation for further processing. For writing the data to
an external entity, the reverse function characters_to_binary/3
comes in handy.
Option unicode is an alias for utf8, as this is the preferred
encoding for Unicode characters in binaries. utf16 is an alias
for {utf16,big} and utf32 is an alias for {utf32,big}. The atoms
big and little denote big- or little-endian encoding.
If the data cannot be converted, either because of illegal
Unicode/ISO Latin-1 characters in the list, or because of
invalid UTF encoding in any binaries, an error tuple is
returned. The error tuple contains the tag error, a list
representing the characters that could be converted before the
error occurred and a representation of the characters including
and after the offending integer/bytes. The last part is mostly
for debugging, as it still constitutes a possibly deep or mixed
list, or both, not necessarily of the same depth as the original
data. The error occurs when traversing the list and whatever is
left to decode is returned "as is".
However, if the input Data is a pure binary, the third part of
the error tuple is guaranteed to be a binary as well.
Errors occur for the following reasons:
* Integers out of range.
If InEncoding is latin1, an error occurs whenever an integer
> 255 is found in the lists.
If InEncoding is of a Unicode type, an error occurs whenever
either of the following is found:
* An integer > 16#10FFFF (the maximum Unicode character)
* An integer in the range 16#D800 to 16#DFFF (invalid range
reserved for UTF-16 surrogate pairs)
* Incorrect UTF encoding.
If InEncoding is one of the UTF types, the bytes in any
binaries must be valid in that encoding.
Errors can occur for various reasons, including the
following:
* "Pure" decoding errors (like the upper bits of the bytes
being wrong).
* The bytes are decoded to a too large number.
* The bytes are decoded to a code point in the invalid
Unicode range.
* Encoding is "overlong", meaning that a number should have
been encoded in fewer bytes.
The case of a truncated UTF is handled specially, see the
paragraph about incomplete binaries below.
If InEncoding is latin1, binaries are always valid as long
as they contain whole bytes, as each byte falls into the
valid ISO Latin-1 range.
A special type of error is when no actual invalid integers or
bytes are found, but a trailing binary() consists of too few
bytes to decode the last character. This error can occur if
bytes are read from a file in chunks or if binaries in other
ways are split on non-UTF character boundaries. An incomplete
tuple is then returned instead of the error tuple. It consists
of the same parts as the error tuple, but the tag is incomplete
instead of error and the last element is always guaranteed to be
a binary consisting of the first part of a (so far) valid UTF
character.
If one UTF character is split over two consecutive binaries in
the Data, the conversion succeeds. This means that a character
can be decoded from a range of binaries as long as the whole
range is specified as input without errors occurring.
Example:
decode_data(Data) ->
case unicode:characters_to_list(Data,unicode) of
{incomplete,Encoded, Rest} ->
More = get_some_more_data(),
Encoded ++ decode_data([Rest, More]);
{error,Encoded,Rest} ->
handle_error(Encoded,Rest);
List ->
List
end.
However, bit strings that are not whole bytes are not allowed,
so a UTF character must be split along 8-bit boundaries to ever
be decoded.
A badarg exception is thrown for the following cases:
* Any parameters are of the wrong type.
* The list structure is invalid (a number as tail).
* The binaries do not contain whole bytes (bit strings).
encoding_to_bom(InEncoding) -> Bin
Types:
Bin = binary()
A binary() such that byte_size(Bin) >= 4.
InEncoding = encoding()
Creates a UTF Byte Order Mark (BOM) as a binary from the
supplied InEncoding. The BOM is, if supported at all, expected
to be placed first in UTF encoded files or messages.
The function returns <<>> for latin1 encoding, as there is no
BOM for ISO Latin-1.
Notice that the BOM for UTF-8 is seldom used, and it is really
not a byte order mark. There are obviously no byte order issues
with UTF-8, so the BOM is only there to differentiate UTF-8
encoding from other UTF formats.
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