perluniintro.pod

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=head1 NAME

perluniintro - Perl Unicode introduction

=head1 DESCRIPTION

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

=head2 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 Unicode I<character> is an abstract entity.  It is not bound to any
particular integer width, especially not to the C language C<char>.
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 like C<LATIN CAPITAL LETTER A> or C<GREEK
SMALL LETTER ALPHA> and unique numbers for the characters, in this
case 0x0041 and 0x03B1, respectively.  These unique numbers are called
I<code points>.

The Unicode standard prefers using hexadecimal notation for the code
points.  If numbers like C<0x0041> are unfamiliar to you, take a peek
at a later section, L</"Hexadecimal Notation">.  The Unicode standard
uses the notation C<U+0041 LATIN CAPITAL LETTER A>, to give the
hexadecimal code point and the normative name of the character.

Unicode also defines various I<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
I<base character> (like C<LATIN CAPITAL LETTER A>), followed by one or
more I<modifiers> (like C<COMBINING ACUTE ACCENT>).  This sequence of
base character and modifiers is called a I<combining 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 are I<precomposed> characters.
C<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, various I<normalization
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 as C<0x10000> (or 65536) characters from
C<0x0000> to C<0xFFFF>.  B<This is untrue.>  Since Unicode 2.0 (July
1996), Unicode has been defined all the way up to 21 bits (C<0x10FFFF>),
and since Unicode 3.1 (March 2001), characters have been defined
beyond C<0xFFFF>.  The first C<0x10000> characters are called the
I<Plane 0>, or the I<Basic 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.  B<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 called I<scripts>,
which is more useful: there is C<Latin> script, C<Greek> script, and
so on.  Scripts usually span varied parts of several blocks.
For further information see L<Unicode::UCD>.

The Unicode code points are just abstract numbers.  To input and
output these abstract numbers, the numbers must be I<encoded> or
I<serialised> somehow.  Unicode defines several I<character encoding
forms>, of which I<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 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 what
I<surrogates> and I<byte order marks> (BOMs) are--see L<perlunicode>.

=head2 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.

B<Starting from Perl 5.8.0, the use of C<use utf8> is no longer
necessary.> In earlier releases the C<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, instead of being attached to the
operations.  Only one case remains where an explicit C<use 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 saying C<use utf8>.  This is not the default because
scripts with legacy 8-bit data in them would break.  See L<utf8>.

=head2 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 are C<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 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 C<:encoding> or C<: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, default C<open()> layer, and C<@ARGV> by using either
the C<-C> command line switch or the C<PERL_UNICODE> environment
variable, see L<perlrun> for the documentation of the C<-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 for
C<useperlio=define>.

=head2 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, see L<perlebcdic> 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".

=head2 Creating Unicode

To create Unicode characters in literals for code points above C<0xFF>,
use the C<\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 use C<chr()>:

    my $hebrew_alef = chr(0x05d0);

See L</"Further Resources"> for how to find all these numeric codes.

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

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

You can also use the C<charnames> 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 also C<pack()> numbers into Unicode
characters:

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

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

If you want to force the result to Unicode characters, use the special
C<"U0"> prefix.  It consumes no arguments but causes the following bytes
to be interpreted as the UTF-8 encoding of Unicode characters:

   my $chars = pack("U0W*", 0x80, 0x42);

Likewise, you can stop such UTF-8 interpretation by using the special
C<"C0"> prefix.

=head2 Handling Unicode

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

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 C<\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:

=head2 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.

The C<Encode> 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

=head2 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