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NAME ^

perluniintro - Perl Unicode introduction

DESCRIPTION ^

This document gives a general idea of Unicode and how to use Unicode in Perl.

Unicode

Unicode is a character set standard with plans to cover all of the writing systems of the world, plus many other symbols.

Unicode and ISO/IEC 10646 are coordinated standards that provide code points for the characters in almost all modern character set standards, covering more than 30 writing systems and hundreds of languages, including all commercially important modern languages. All characters in the largest Chinese, Japanese, and Korean dictionaries are also encoded. The standards will eventually cover almost all characters in more than 250 writing systems and thousands of languages.

A Unicode character is an abstract entity. It is not bound to any particular integer width, and especially not to the C language char. Unicode is language neutral and display neutral: it doesn't encode the language of the text, and it doesn't define fonts or other graphical layout details. Unicode operates on characters and on text built from those characters.

Unicode defines characters like LATIN CAPITAL LETTER A or GREEK SMALL LETTER ALPHA, and then unique numbers for those, hexadecimal 0x0041 or 0x03B1 for those particular characters. Such unique numbers are called code points.

The Unicode standard prefers using hexadecimal notation for the code points. (In case this notation, numbers like 0x0041, is unfamiliar to you, take a peek at a later section, "Hexadecimal Notation".) The Unicode standard uses the notation U+0041 LATIN CAPITAL LETTER A, which gives the hexadecimal code point, and the normative name of the character.

Unicode also defines various properties for the characters, like "uppercase" or "lowercase", "decimal digit", or "punctuation": these properties are independent of the names of the characters. Furthermore, various operations on the characters like uppercasing, lowercasing, and collating (sorting), are defined.

A Unicode character consists either of a single code point, or a base character (like LATIN CAPITAL LETTER A), followed by one or more modifiers (like COMBINING ACUTE ACCENT). This sequence of a base character and modifiers is called a combining character sequence.

Whether to call these combining character sequences, as a whole, "characters" depends on your point of view. If you are a programmer, you probably would tend towards seeing each element in the sequences as one unit, one "character", but from the user viewpoint, the sequence as a whole is probably considered one "character", since that's probably what it looks like in the context of the user's language.

With this "as a whole" view of characters, the number of characters is open-ended. But in the programmer's "one unit is one character" point of view, the concept of "characters" is more deterministic, and so we take that point of view in this document: one "character" is one Unicode code point, be it a base character or a combining character.

For some of the combinations there are precomposed characters, for example LATIN CAPITAL LETTER A WITH ACUTE is defined as a single code point. These precomposed characters are, however, often available only for some combinations, and mainly they are meant to support round-trip conversions between Unicode and legacy standards (like the ISO 8859), and in general case the composing method is more extensible. To support conversion between the different compositions of the characters, various normalization forms are also defined.

Because of backward compatibility with legacy encodings, the "a unique number for every character" breaks down a bit: "at least one number for every character" is closer to truth. (This happens when the same character has been encoded in several legacy encodings.) The converse is also not true: not every code point has an assigned character. Firstly, there are unallocated code points within otherwise used blocks. Secondly, there are special Unicode control characters that do not represent true characters.

A common myth about Unicode is that it would be "16-bit", that is, 0x10000 (or 65536) characters from 0x0000 to 0xFFFF. This is untrue. Since Unicode 2.0 Unicode has been defined all the way up to 21 bits (0x10FFFF), and since 3.1 characters have been defined beyond 0xFFFF. The first 0x10000 characters are called the Plane 0, or the Basic Multilingual Plane (BMP). With the Unicode 3.1, 17 planes in all are defined (but nowhere near full of defined characters yet).

Another myth is that the 256-character blocks have something to do with languages: a block per language. Also this is untrue. The division into the blocks exists but it is almost completely accidental, an artifact of how the characters have been historically allocated. Instead, there is a concept called scripts, which may be more useful: there is Latin script, Greek script, and so on. Scripts usually span several parts of several blocks. For further information see Unicode::UCD.

The Unicode code points are just abstract numbers. To input and output these abstract numbers, the numbers must be encoded somehow. Unicode defines several character encoding forms, of which UTF-8 is perhaps the most popular. UTF-8 is a variable length encoding that encodes Unicode characters as 1 to 6 bytes (only 4 with the currently defined characters). Other encodings include UTF-16 and UTF-32 and their big and little endian variants (UTF-8 is byteorder independent). The ISO/IEC 10646 defines the UCS-2 and UCS-4 encoding forms.

For more information about encodings, for example to learn what surrogates and byte order marks (BOMs) are, see perlunicode.

Perl's Unicode Support

Starting from Perl 5.6.0, Perl has had the capability of handling Unicode natively. The first recommended release for serious Unicode work is Perl 5.8.0, however. The maintenance release 5.6.1 fixed many of the problems of the initial implementation of Unicode, but for example regular expressions didn't really work with Unicode.

Starting from Perl 5.8.0, the use of use utf8 is no longer necessary. In earlier releases the utf8 pragma was used to declare that operations in the current block or file would be Unicode-aware. This model was found to be wrong, or at least clumsy: the Unicodeness is now carried with the data, not attached to the operations. (There is one remaining case where an explicit use utf8 is needed: if your Perl script itself is encoded in UTF-8, you can use UTF-8 in your variable and subroutine names, and in your string and regular expression literals, by saying use utf8. This is not the default because that would break existing scripts having legacy 8-bit data in them.)

Perl's Unicode Model

Perl supports both the old, pre-5.6, model of strings of eight-bit native bytes, and strings of Unicode characters. The principle is that Perl tries to keep its data as eight-bit bytes for as long as possible, but as soon as Unicodeness cannot be avoided, the data is transparently upgraded to Unicode.

Internally, Perl currently uses either whatever the native eight-bit character set of the platform (for example Latin-1) or UTF-8 to encode Unicode strings. Specifically, if all code points in the string are 0xFF or less, Perl uses the native eight-bit character set. Otherwise, it uses UTF-8.

A user of Perl does not normally need to know nor care how Perl happens to encode its internal strings, but it becomes relevant when outputting Unicode strings to a stream without a discipline (one with the "default default"). In such a case, the raw bytes used internally (the native character set or UTF-8, as appropriate for each string) will be used, and a "Wide character" warning will be issued if those strings contain a character beyond 0x00FF.

For example,

      perl -e 'print "\x{DF}\n", "\x{0100}\x{DF}\n"'              

produces a fairly useless mixture of native bytes and UTF-8, as well as a warning.

To output UTF-8 always, use the ":utf8" output discipline. Prepending

      binmode(STDOUT, ":utf8");

to this sample program ensures the output is completely UTF-8, and of course, removes the warning.

Perl 5.8.0 also supports Unicode on EBCDIC platforms. There, the support is somewhat harder to implement since additional conversions are needed at every step. Because of these difficulties, the Unicode support isn't quite as full as in other, mainly ASCII-based, platforms (the Unicode support is better than in the 5.6 series, which didn't work much at all for EBCDIC platform). On EBCDIC platforms, the internal Unicode encoding form is UTF-EBCDIC instead of UTF-8 (the difference is that as UTF-8 is "ASCII-safe" in that ASCII characters encode to UTF-8 as-is, UTF-EBCDIC is "EBCDIC-safe").

Creating Unicode

To create Unicode characters in literals for code points above 0xFF, use the \x{...} notation in doublequoted strings:

    my $smiley = "\x{263a}";

Similarly in regular expression literals

    $smiley =~ /\x{263a}/;

At run-time you can use chr():

    my $hebrew_alef = chr(0x05d0);

(See "Further Resources" for how to find all these numeric codes.)

Naturally, ord() will do the reverse: turn a character to a code point.

Note that \x.. (no {} and only two hexadecimal digits), \x{...}, and chr(...) for arguments less than 0x100 (decimal 256) generate an eight-bit character for backward compatibility with older Perls. For arguments of 0x100 or more, Unicode characters are always produced. If you want to force the production of Unicode characters regardless of the numeric value, use pack("U", ...) instead of \x.., \x{...}, or chr().

You can also use the charnames pragma to invoke characters by name in doublequoted strings:

    use charnames ':full';
    my $arabic_alef = "\N{ARABIC LETTER ALEF}";

And, as mentioned above, you can also pack() numbers into Unicode characters:

   my $georgian_an  = pack("U", 0x10a0);

Note that both \x{...} and \N{...} are compile-time string constants: you cannot use variables in them. if you want similar run-time functionality, use chr() and charnames::vianame().

Also note that if all the code points for pack "U" are below 0x100, bytes will be generated, just like if you were using chr().

   my $bytes = pack("U*", 0x80, 0xFF);

If you want to force the result to Unicode characters, use the special "U0" prefix. It consumes no arguments but forces the result to be in Unicode characters, instead of bytes.

   my $chars = pack("U0U*", 0x80, 0xFF);

Handling Unicode

Handling Unicode is for the most part transparent: just use the strings as usual. Functions like index(), length(), and substr() will work on the Unicode characters; regular expressions will work on the Unicode characters (see perlunicode and perlretut).

Note that Perl does not consider combining character sequences to be characters, such for example

    use charnames ':full';
    print length("\N{LATIN CAPITAL LETTER A}\N{COMBINING ACUTE ACCENT}"), "\n";

will print 2, not 1. The only exception is that regular expressions have \X for matching a combining character sequence.

When life is not quite so transparent is working with legacy encodings, and I/O, and certain special cases.

Legacy Encodings

When you combine legacy data and Unicode the legacy data needs to be upgraded to Unicode. Normally ISO 8859-1 (or EBCDIC, if applicable) is assumed. You can override this assumption by using the encoding pragma, for example

    use encoding 'latin2'; # ISO 8859-2

in which case literals (string or regular expression) and chr/ord in your whole script are assumed to produce Unicode characters from ISO 8859-2 code points. Note that the matching for the encoding names is forgiving: instead of latin2 you could have said Latin 2, or iso8859-2, and so forth. With just

    use encoding;

first the environment variable PERL_ENCODING will be consulted, and if that doesn't exist, ISO 8859-1 (Latin 1) will be assumed.

The Encode module knows about many encodings and it has interfaces for doing conversions between those encodings:

    use Encode 'from_to';
    from_to($data, "iso-8859-3", "utf-8"); # from legacy to utf-8

Unicode I/O

Normally, writing out Unicode data

    print FH $some_string_with_unicode, "\n";

produces raw bytes that Perl happens to use to internally encode the Unicode string (which depends on the system, as well as what characters happen to be in the string at the time). If any of the characters are at code points 0x100 or above, you will get a warning. To ensure that the output is explicitly rendered in the encoding you desire (and to avoid the warning), open the stream with the desired encoding. Some examples:

    open FH, ">:ucs2",      "file"
    open FH, ">:utf8",      "file";
    open FH, ">:Shift-JIS", "file";

and on already open streams use binmode():

    binmode(STDOUT, ":ucs2");
    binmode(STDOUT, ":utf8");
    binmode(STDOUT, ":Shift-JIS");

See documentation for the Encode module for many supported encodings.

Reading in a file that you know happens to be encoded in one of the Unicode encodings does not magically turn the data into Unicode in Perl's eyes. To do that, specify the appropriate discipline when opening files

    open(my $fh,'<:utf8', 'anything');
    my $line_of_unicode = <$fh>;

    open(my $fh,'<:Big5', 'anything');
    my $line_of_unicode = <$fh>;

The I/O disciplines can also be specified more flexibly with the open pragma; see open:

    use open ':utf8'; # input and output default discipline will be UTF-8
    open X, ">file";
    print X chr(0x100), "\n";
    close X;
    open Y, "<file";
    printf "%#x\n", ord(<Y>); # this should print 0x100
    close Y;

With the open pragma you can use the :locale discipline

    $ENV{LC_ALL} = $ENV{LANG} = 'ru_RU.KOI8-R';
    # the :locale will probe the locale environment variables like LC_ALL
    use open OUT => ':locale'; # russki parusski
    open(O, ">koi8");
    print O chr(0x430); # Unicode CYRILLIC SMALL LETTER A = KOI8-R 0xc1
    close O;
    open(I, "<koi8");
    printf "%#x\n", ord(<I>), "\n"; # this should print 0xc1
    close I;

or you can also use the ':encoding(...)' discipline

    open(my $epic,'<:encoding(iso-8859-7)','iliad.greek');
    my $line_of_unicode = <$epic>;

These methods install a transparent filter on the I/O stream that converts data from the specified encoding when it is read in from the stream. The result is always Unicode.

The open pragma affects all the open() calls after the pragma by setting default disciplines. If you want to affect only certain streams, use explicit disciplines directly in the open() call.

You can switch encodings on an already opened stream by using binmode(); see "binmode" in perlfunc.

The :locale does not currently (as of Perl 5.8.0) work with open() and binmode(), only with the open pragma. The :utf8 and :encoding(...) methods do work with all of open(), binmode(), and the open pragma.

Similarly, you may use these I/O disciplines on output streams to automatically convert Unicode to the specified encoding when it is written to the stream. For example, the following snippet copies the contents of the file "text.jis" (encoded as ISO-2022-JP, aka JIS) to the file "text.utf8", encoded as UTF-8:

    open(my $nihongo, '<:encoding(iso2022-jp)', 'text.jis');
    open(my $unicode, '>:utf8',                 'text.utf8');
    while (<$nihongo>) { print $unicode }

The naming of encodings, both by the open() and by the open pragma, is similarly understanding as with the encoding pragma: koi8-r and KOI8R will both be understood.

Common encodings recognized by ISO, MIME, IANA, and various other standardisation organisations are recognised; for a more detailed list see Encode.

read() reads characters and returns the number of characters. seek() and tell() operate on byte counts, as do sysread() and sysseek().

Notice that because of the default behaviour of not doing any conversion upon input if there is no default discipline, it is easy to mistakenly write code that keeps on expanding a file by repeatedly encoding:

    # BAD CODE WARNING
    open F, "file";
    local $/; ## read in the whole file of 8-bit characters
    $t = <F>;
    close F;
    open F, ">:utf8", "file";
    print F $t; ## convert to UTF-8 on output
    close F;

If you run this code twice, the contents of the file will be twice UTF-8 encoded. A use open ':utf8' would have avoided the bug, or explicitly opening also the file for input as UTF-8.

NOTE: the :utf8 and :encoding features work only if your Perl has been built with the new "perlio" feature. Almost all Perl 5.8 platforms do use "perlio", though: you can see whether yours is by running "perl -V" and looking for useperlio=define.

Displaying Unicode As Text

Sometimes you might want to display Perl scalars containing Unicode as simple ASCII (or EBCDIC) text. The following subroutine converts its argument so that Unicode characters with code points greater than 255 are displayed as "\x{...}", control characters (like "\n") are displayed as "\x..", and the rest of the characters as themselves:

   sub nice_string {
       join("",
         map { $_ > 255 ?                  # if wide character...
               sprintf("\\x{%04X}", $_) :  # \x{...}
               chr($_) =~ /[[:cntrl:]]/ ?  # else if control character ...
               sprintf("\\x%02X", $_) :    # \x..
               chr($_)                     # else as themselves
         } unpack("U*", $_[0]));           # unpack Unicode characters
   }

For example,

   nice_string("foo\x{100}bar\n")

returns:

   "foo\x{0100}bar\x0A"

Special Cases

Advanced Topics

Miscellaneous

Questions With Answers

Hexadecimal Notation

The Unicode standard prefers using hexadecimal notation because that shows better the division of Unicode into blocks of 256 characters. Hexadecimal is also simply shorter than decimal. You can use decimal notation, too, but learning to use hexadecimal just makes life easier with the Unicode standard.

The 0x prefix means a hexadecimal number, the digits are 0-9 and a-f (or A-F, case doesn't matter). Each hexadecimal digit represents four bits, or half a byte. print 0x..., "\n" will show a hexadecimal number in decimal, and printf "%x\n", $decimal will show a decimal number in hexadecimal. If you have just the "hexdigits" of a hexadecimal number, you can use the hex() function.

    print 0x0009, "\n";    # 9
    print 0x000a, "\n";    # 10
    print 0x000f, "\n";    # 15
    print 0x0010, "\n";    # 16
    print 0x0011, "\n";    # 17
    print 0x0100, "\n";    # 256

    print 0x0041, "\n";    # 65

    printf "%x\n",  65;    # 41
    printf "%#x\n", 65;    # 0x41

    print hex("41"), "\n"; # 65

Further Resources

UNICODE IN OLDER PERLS ^

If you cannot upgrade your Perl to 5.8.0 or later, you can still do some Unicode processing by using the modules Unicode::String, Unicode::Map8, and Unicode::Map, available from CPAN. If you have the GNU recode installed, you can also use the Perl frontend Convert::Recode for character conversions.

The following are fast conversions from ISO 8859-1 (Latin-1) bytes to UTF-8 bytes, the code works even with older Perl 5 versions.

    # ISO 8859-1 to UTF-8
    s/([\x80-\xFF])/chr(0xC0|ord($1)>>6).chr(0x80|ord($1)&0x3F)/eg;

    # UTF-8 to ISO 8859-1
    s/([\xC2\xC3])([\x80-\xBF])/chr(ord($1)<<6&0xC0|ord($2)&0x3F)/eg;

SEE ALSO ^

perlunicode, Encode, encoding, open, utf8, bytes, perlretut, Unicode::Collate, Unicode::Normalize, Unicode::UCD

ACKNOWLEDGEMENTS ^

Thanks to the kind readers of the perl5-porters@perl.org, perl-unicode@perl.org, linux-utf8@nl.linux.org, and unicore@unicode.org mailing lists for their valuable feedback.

AUTHOR, COPYRIGHT, AND LICENSE ^

Copyright 2001-2002 Jarkko Hietaniemi <jhi@iki.fi>

This document may be distributed under the same terms as Perl itself.

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