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perluniintro
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CONTENTS

#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 which plans to codify 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 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. Unicode 1.0 was released in October 1991, and 4.0 in April 2003.

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

Unicode defines characters likeLATIN CAPITAL LETTER A orGREEK SMALL LETTER ALPHA and unique numbers for the characters, in this case 0x0041 and 0x03B1, respectively. These unique numbers are calledcode points.

The Unicode standard prefers using hexadecimal notation for the code points. If numbers like0x0041 are unfamiliar to you, take a peek at a later section,"Hexadecimal Notation". The Unicode standard uses the notationU+0041 LATIN CAPITAL LETTER A, to give the hexadecimal code point and the normative name of the character.

Unicode also defines variousproperties 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 abase character (likeLATIN CAPITAL LETTER A), followed by one or moremodifiers (likeCOMBINING ACUTE ACCENT). This sequence of base character and modifiers is called acombining character sequence.

Whether to call these combining character sequences "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, or "character". The whole sequence could be seen as one "character", however, from the user's point of view, since that's probably what it looks like in the context of the user's language.

With this "whole sequence" view of characters, the total 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. In this document, we take that second point of view: one "character" is one Unicode code point, be it a base character or a combining character.

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

Because of backward compatibility with legacy encodings, the "a unique number for every character" idea breaks down a bit: instead, there is "at least one number for every character". The same character could be represented differently in several legacy encodings. The converse is also not true: some code points do not have 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, Unicode is only represented as0x10000 (or 65536) characters from0x0000 to0xFFFF.This is untrue. Since Unicode 2.0 (July 1996), Unicode has been defined all the way up to 21 bits (0x10FFFF), and since Unicode 3.1 (March 2001), characters have been defined beyond0xFFFF. The first0x10000 characters are called thePlane 0, or theBasic Multilingual Plane (BMP). With Unicode 3.1, 17 (yes, seventeen) planes in all were defined--but they are nowhere near full of defined characters, yet.

Another myth is that the 256-character blocks have something to do with languages--that each block would define the characters used by a language or a set of languages.This is also untrue. The division into blocks exists, but it is almost completely accidental--an artifact of how the characters have been and still are allocated. Instead, there is a concept calledscripts, which is more useful: there isLatin script,Greek script, and so on. Scripts usually span varied parts of several blocks. For further information seeUnicode::UCD.

The Unicode code points are just abstract numbers. To input and output these abstract numbers, the numbers must beencoded orserialised somehow. Unicode defines severalcharacter encoding forms, of whichUTF-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 byte-order independent) The ISO/IEC 10646 defines the UCS-2 and UCS-4 encoding forms.

For more information about encodings--for instance, to learn whatsurrogates andbyte order marks (BOMs) are--seeperlunicode.

#Perl's Unicode Support

Starting from Perl 5.6.0, Perl has had the capacity to handle Unicode natively. Perl 5.8.0, however, is the first recommended release for serious Unicode work. The maintenance release 5.6.1 fixed many of the problems of the initial Unicode implementation, but for example regular expressions still do not work with Unicode in 5.6.1.

Starting from Perl 5.8.0, the use ofuse utf8 is no longer necessary. In earlier releases theutf8 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, instead of being attached to the operations. Only one case remains where an explicituse utf8 is needed: if your Perl script itself is encoded in UTF-8, you can use UTF-8 in your identifier names, and in string and regular expression literals, by sayinguse utf8. This is not the default because scripts with legacy 8-bit data in them would break. Seeutf8.

#Perl's Unicode Model

Perl supports both pre-5.6 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) is, defaulting to UTF-8, to encode Unicode strings. Specifically, if all code points in the string are0xFF 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 PerlIO layer -- one with the "default" encoding. 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:

Wide character in print at ...

To output UTF-8, use the:utf8 output layer. Prepending

binmode(STDOUT, ":utf8");

to this sample program ensures that the output is completely UTF-8, and removes the program's warning.

You can enable automatic UTF-8-ification of your standard file handles, defaultopen() layer, and@ARGV by using either the-C command line switch or thePERL_UNICODE environment variable, seeperlrun for the documentation of the-C switch.

Note that this means that Perl expects other software to work, too: if Perl has been led to believe that STDIN should be UTF-8, but then STDIN coming in from another command is not UTF-8, Perl will complain about the malformed UTF-8.

All features that combine Unicode and I/O also require using 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 foruseperlio=define.

#Unicode and EBCDIC

Perl 5.8.0 also supports Unicode on EBCDIC platforms. There, Unicode support is somewhat more complex to implement since additional conversions are needed at every step. Some problems remain, seeperlebcdic for details.

In any case, the Unicode support on EBCDIC platforms 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, while UTF-EBCDIC is "EBCDIC-safe".

#Creating Unicode

To create Unicode characters in literals for code points above0xFF, use the\x{...} notation in double-quoted strings:

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

Similarly, it can be used in regular expression literals

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

At run-time you can usechr():

my $hebrew_alef = chr(0x05d0);

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

Naturally,ord() will do the reverse: it turns a character into a code point.

Note that\x.. (no{} and only two hexadecimal digits),\x{...}, andchr(...) for arguments less than0x100 (decimal 256) generate an eight-bit character for backward compatibility with older Perls. For arguments of0x100 or more, Unicode characters are always produced. If you want to force the production of Unicode characters regardless of the numeric value, usepack("U", ...) instead of\x..,\x{...}, orchr().

You can also use thecharnames pragma to invoke characters by name in double-quoted strings:

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

And, as mentioned above, you can alsopack() 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, usechr() andcharnames::vianame().

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("U0C*", 0x80, 0x42);

Likewise, you can force the result to be bytes by using the special"C0" prefix.

#Handling Unicode

Handling Unicode is for the most part transparent: just use the strings as usual. Functions likeindex(),length(), andsubstr() will work on the Unicode characters; regular expressions will work on the Unicode characters (seeperlunicode andperlretut).

Note that Perl considers combining character sequences to be separate characters, so 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.

Life is not quite so transparent, however, when working with legacy encodings, 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 theencoding pragma, for example

use encoding 'latin2'; # ISO 8859-2

in which case literals (string or regular expressions),chr(), andord() in your whole script are assumed to produce Unicode characters from ISO 8859-2 code points. Note that the matching for encoding names is forgiving: instead oflatin2 you could have saidLatin 2, oriso8859-2, or other variations. With just

use encoding;

the environment variablePERL_ENCODING will be consulted. If that variable isn't set, the encoding pragma will fail.

TheEncode module knows about many encodings and has interfaces for doing conversions between those encodings:

use Encode 'decode';$data = decode("iso-8859-3", $data); # convert 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. Perl's internal encoding 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 points0x100 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, ">:utf8", "file";open FH, ">:encoding(ucs2)",      "file";open FH, ">:encoding(UTF-8)",     "file";open FH, ">:encoding(shift_jis)", "file";

and on already open streams, usebinmode():

binmode(STDOUT, ":utf8");binmode(STDOUT, ":encoding(ucs2)");binmode(STDOUT, ":encoding(UTF-8)");binmode(STDOUT, ":encoding(shift_jis)");

The matching of encoding names is loose: case does not matter, and many encodings have several aliases. Note that the:utf8 layer must always be specified exactly like that; it isnot subject to the loose matching of encoding names.

SeePerlIO for the:utf8 layer,PerlIO::encoding andEncode::PerlIO for the:encoding() layer, andEncode::Supported for many encodings supported by theEncode module.

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

open(my $fh,'<:utf8', 'anything');my $line_of_unicode = <$fh>;open(my $fh,'<:encoding(Big5)', 'anything');my $line_of_unicode = <$fh>;

The I/O layers can also be specified more flexibly with theopen pragma. Seeopen, or look at the following example.

use open ':utf8'; # input and output default layer will be UTF-8open X, ">file";print X chr(0x100), "\n";close X;open Y, "<file";printf "%#x\n", ord(<Y>); # this should print 0x100close Y;

With theopen pragma you can use the:locale layer

BEGIN { $ENV{LC_ALL} = $ENV{LANG} = 'ru_RU.KOI8-R' }# the :locale will probe the locale environment variables like LC_ALLuse open OUT => ':locale'; # russki parusskiopen(O, ">koi8");print O chr(0x430); # Unicode CYRILLIC SMALL LETTER A = KOI8-R 0xc1close O;open(I, "<koi8");printf "%#x\n", ord(<I>), "\n"; # this should print 0xc1close I;

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

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.

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

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

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

Similarly, you may use these I/O layers 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(iso-2022-jp)', 'text.jis');open(my $unicode, '>:utf8',                  'text.utf8');while (<$nihongo>) { print $unicode $_ }

The naming of encodings, both by theopen() and by theopen pragma, is similar to theencoding pragma in that it allows for flexible names:koi8-r andKOI8R will both be understood.

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

read() reads characters and returns the number of characters.seek() andtell() operate on byte counts, as dosysread() andsysseek().

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

# BAD CODE WARNINGopen 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 outputclose F;

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

NOTE: the:utf8 and:encoding features work only if your Perl has been built with the new PerlIO feature (which is the default on most systems).

#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", $_) :    # \x..            quotemeta(chr($_))          # else quoted or as themselves      } unpack("U*", $_[0]));           # unpack Unicode characters}

For example,

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

returns the string

'foo\x{0100}bar\x0A'

which is ready to be printed.

#Special Cases

#Advanced Topics

#Miscellaneous

#Questions With Answers

#Hexadecimal Notation

The Unicode standard prefers using hexadecimal notation because that more clearly shows 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. TheU+HHHH notation uses hexadecimal, for example.

The0x prefix means a hexadecimal number, the digits are 0-9and 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, andprintf "%x\n", $decimal will show a decimal number in hexadecimal. If you have just the "hex digits" of a hexadecimal number, you can use thehex() function.

print 0x0009, "\n";    # 9print 0x000a, "\n";    # 10print 0x000f, "\n";    # 15print 0x0010, "\n";    # 16print 0x0011, "\n";    # 17print 0x0100, "\n";    # 256print 0x0041, "\n";    # 65printf "%x\n",  65;    # 41printf "%#x\n", 65;    # 0x41print 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 modulesUnicode::String,Unicode::Map8, andUnicode::Map, available from CPAN. If you have the GNU recode installed, you can also use the Perl front-endConvert::Recode for character conversions.

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

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

#SEE ALSO

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

#ACKNOWLEDGMENTS

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.

Perldoc Browser is maintained by Dan Book (DBOOK). Please contact him via theGitHub issue tracker oremail regarding any issues with the site itself, search, or rendering of documentation.

The Perl documentation is maintained by the Perl 5 Porters in the development of Perl. Please contact them via thePerl issue tracker, themailing list, orIRC to report any issues with the contents or format of the documentation.

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