Functions for converting Unicode characters

This module contains functions for converting between different character representations. Basically it converts between iso-latin-1 characters and Unicode ditto, 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 codepoint for the character.

Other Unicode encodings than integers representing codepoints 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. Latin1 encoding is supported both for backward compatibility and for communication with external entities not supporting Unicode character sets.


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.

unicode_char() = integer() >= 0

An integer() representing a valid unicode codepoint.

chardata() = charlist() | unicode_binary()

charlist() = [unicode_char() | unicode_binary() | charlist()]

A unicode_binary is allowed as the tail of the list.

external_unicode_binary() = binary()

A binary() with characters coded in a user specified Unicode encoding other than UTF-8 (UTF-16 or UTF-32).

external_chardata() =
            external_charlist() | external_unicode_binary()

external_charlist() =
            [unicode_char() |
             external_unicode_binary() |

An external_unicode_binary() is allowed as the tail of the list.

latin1_binary() = binary()

A binary() with characters coded in iso-latin-1.

latin1_char() = byte()

An integer() representing valid latin1 character (0-255).

latin1_chardata() = latin1_charlist() | latin1_binary()

latin1_charlist() =
            [latin1_char() | latin1_binary() | latin1_charlist()]

A latin1_binary() is allowed as the tail of the list.


bom_to_encoding(Bin) -> {Encoding, Length}

  • Bin = binary()
  • Encoding =
        latin1 | utf8 | {utf16, endian()} | {utf32, endian()}
  • Length = integer() >= 0
  • endian() = big | little
Bin = A binary() of byte_size 4 or more.

Check for a UTF byte order mark (BOM) in the beginning of a binary. If the supplied binary Bin begins with a valid byte order mark for either UTF-8, UTF-16 or UTF-32, the function returns the encoding identified along with the length of the BOM in bytes.

If no BOM is found, the function returns {latin1,0}

characters_to_list(Data) -> Result

Same as characters_to_list(Data,unicode).

characters_to_list(Data, InEncoding) -> Result

This function converts a possibly deep list of integers and binaries into a list of integers representing unicode characters. The binaries in the input may have characters encoded as latin1 (0 - 255, one character per byte), in which case the InEncoding parameter should be given as latin1, or have characters encoded as one of the UTF-encodings, which is given as the InEncoding parameter. Only when the InEncoding is one of the UTF encodings, integers in the list are allowed to be grater than 255.

If InEncoding is latin1, the Data parameter corresponds to the iodata() type, but for unicode, the Data parameter can contain integers greater than 255 (unicode characters beyond the iso-latin-1 range), which would make it invalid as iodata().

The purpose of the function is mainly to be able 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.

The 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 big and little atoms denote big or little endian encoding.

If for some reason, the data cannot be converted, either because of illegal unicode/latin1 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 and/or mixed list, not necessarily of the same depth as the original data. The error occurs when traversing the list and whatever's left to decode is simply 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 greater than 255 is found in the lists. If InEncoding is of a Unicode type, an error occurs whenever an integer greater than 16#10FFFF (the maximum unicode character), in the range 16#D800 to 16#DFFF (invalid range reserved for UTF-16 surrogate pairs) is found. UTF encoding incorrect - If InEncoding is one of the UTF types, the bytes in any binaries have to be valid in that encoding. Errors can occur for various reasons, including "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 or 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 might occur if bytes are read from a file in chunks or binaries in other ways are split on non UTF character boundaries. In this case an incomplete tuple is 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 characters 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 given 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.

Bit-strings that are not whole bytes are however not allowed, so a UTF character has to be split along 8-bit boundaries to ever be decoded.

If any parameters are of the wrong type, the list structure is invalid (a number as tail) or the binaries does not contain whole bytes (bit-strings), a badarg exception is thrown.

characters_to_binary(Data) -> Result

Same as characters_to_binary(Data, unicode, unicode).

characters_to_binary(Data,InEncoding) -> Result

Same as characters_to_binary(Data, InEncoding, unicode).

characters_to_binary(Data, InEncoding, OutEncoding) -> Result

This function behaves as characters_to_list/2, but produces an binary instead of a unicode list. The InEncoding defines how input is to be interpreted if binaries are present in the Data, while OutEncoding defines in what format output is to be generated.

The 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 big and little atoms denote big or little endian encoding.

Errors and exceptions occur as in characters_to_list/2, but the second element in the error or incomplete tuple will be a binary() and not a list().

encoding_to_bom(InEncoding) -> Bin

Bin = A binary() of byte_size 4 or more.

Create an 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 the latin1 encoding, there is no BOM for ISO-latin-1.

It can be noted 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.