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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 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 I<character> is an abstract entity.  It is not bound to any
particular integer width, and especially not to the C language C<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 C<LATIN CAPITAL LETTER A> or C<GREEK
SMALL LETTER ALPHA>, and then unique numbers for those, hexadecimal
0x0041 or 0x03B1 for those particular characters.  Such unique
numbers are called I<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, L</"Hexadecimal Notation">.)
The Unicode standard uses the notation C<U+0041 LATIN CAPITAL LETTER A>,
which gives 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
a base character and modifiers is called a I<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 I<precomposed> characters,
for example C<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 I<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.  B<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 I<Plane 0>, or the I<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.  B<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 I<scripts>, which may
be more useful: there is C<Latin> script, C<Greek> script, and so on.
Scripts usually span several 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> 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 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
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 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.

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, not attached to the operations.  (There
is one remaining case 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
variable and subroutine names, and in your string and regular
expression literals, by saying C<use utf8>.  This is not the default
because that would break existing scripts having legacy 8-bit data in
them.)

=head2 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.

If your locale environment variables (LANGUAGE, LC_ALL, LC_CTYPE, LANG)
contain the strings 'UTF-8' or 'UTF8' (case-insensitive matching),
the default encoding of your STDIN, STDOUT, and STDERR, and of
B<any subsequent file open>, is UTF-8.

=head2 Unicode and EBCDIC

Perl 5.8.0 also supports Unicode on EBCDIC platforms.  There,
the 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, UTF-EBCDIC is
"EBCDIC-safe").

=head2 Creating Unicode

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

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

Similarly 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: turn a character to a code point.

Note that C<\x..> (no C<{}> and only two hexadecimal digits),
C<\x{...}>, and C<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 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 doublequoted 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()>.

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

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

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

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

=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 does B<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 C<\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.

=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.  You can override this assumption by
using the C<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 C<latin2> you could have said 
C<Latin 2>, or C<iso8859-2>, and so forth.  With just

    use encoding;

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

The C<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

=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 (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 C<binmode()>:

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

See documentation for the C<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 C<open> pragma; see L<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 C<open> pragma you can use the C<: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 C<':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 L<open> pragma affects all the C<open()> calls after the pragma by
setting default disciplines.  If you want to affect only certain
streams, use explicit disciplines directly in the C<open()> call.

You can switch encodings on an already opened stream by using
C<binmode()>; see L<perlfunc/binmode>.

The C<:locale> does not currently (as of Perl 5.8.0) work with
C<open()> and C<binmode()>, only with the C<open> pragma.  The
C<:utf8> and C<:encoding(...)> methods do work with all of C<open()>,
C<binmode()>, and the C<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 C<open()> and by the C<open>
pragma, is similarly understanding as with the C<encoding> pragma:
C<koi8-r> and C<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 L<Encode>.

C<read()> reads characters and returns the number of characters.
C<seek()> and C<tell()> operate on byte counts, as do C<sysread()>
and C<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 F<file> will be twice
UTF-8 encoded.  A C<use open ':utf8'> would have avoided the bug, or
explicitly opening also the F<file> for input as UTF-8.

In some filesystems (for example Microsoft NTFS and Apple HFS+) the
filenames are in UTF-8 .  By using opendir() and File::Glob you can
make readdir() and glob() to return the filenames as Unicode, see
L<perlfunc/opendir> and L<File::Glob> for details.

B<NOTE>: the C<:utf8> and C<: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 C<useperlio=define>.

=head2 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"

=head2 Special Cases

=over 4

=item *

Bit Complement Operator ~ And vec()

The bit complement operator C<~> may produce surprising results if used on
strings containing characters with ordinal values above 255. In such a
case, the results are consistent with the internal encoding of the
characters, but not with much else. So don't do that. Similarly for vec():
you will be operating on the internally encoded bit patterns of the Unicode
characters, not on the code point values, which is very probably not what
you want.

=item *

Peeking At Perl's Internal Encoding

Normal users of Perl should never care how Perl encodes any particular
Unicode string (because the normal ways to get at the contents of a
string with Unicode -- via input and output -- should always be via
explicitly-defined I/O disciplines). But if you must, there are two
ways of looking behind the scenes.

One way of peeking inside the internal encoding of Unicode characters
is to use C<unpack("C*", ...> to get the bytes, or C<unpack("H*", ...)>
to display the bytes:

    # this prints  c4 80  for the UTF-8 bytes 0xc4 0x80
    print join(" ", unpack("H*", pack("U", 0x100))), "\n";

Yet another way would be to use the Devel::Peek module:

    perl -MDevel::Peek -e 'Dump(chr(0x100))'

That shows the UTF8 flag in FLAGS and both the UTF-8 bytes
and Unicode characters in PV.  See also later in this document
the discussion about the C<is_utf8> function of the C<Encode> module.

=back

=head2 Advanced Topics

=over 4

=item *

String Equivalence

The question of string equivalence turns somewhat complicated
in Unicode: what do you mean by equal?

(Is C<LATIN CAPITAL LETTER A WITH ACUTE> equal to
C<LATIN CAPITAL LETTER A>?)

The short answer is that by default Perl compares equivalence (C<eq>,
C<ne>) based only on code points of the characters.  In the above
case, the answer is no (because 0x00C1 != 0x0041).  But sometimes any
CAPITAL LETTER As being considered equal, or even any As of any case,
would be desirable.

The long answer is that you need to consider character normalization
and casing issues: see L<Unicode::Normalize>, and Unicode Technical
Reports #15 and #21, I<Unicode Normalization Forms> and I<Case
Mappings>, http://www.unicode.org/unicode/reports/tr15/
http://www.unicode.org/unicode/reports/tr21/

As of Perl 5.8.0, regular expression case-ignoring matching
implements only 1:1 semantics: one character matches one character.
In I<Case Mappings> both 1:N and N:1 matches are defined.

=item *

String Collation

People like to see their strings nicely sorted, or as Unicode
parlance goes, collated.  But again, what do you mean by collate?

(Does C<LATIN CAPITAL LETTER A WITH ACUTE> come before or after
C<LATIN CAPITAL LETTER A WITH GRAVE>?)

The short answer is that by default, Perl compares strings (C<lt>,
C<le>, C<cmp>, C<ge>, C<gt>) based only on the code points of the
characters.  In the above case, the answer is "after", since 0x00C1 > 0x00C0.

The long answer is that "it depends", and a good answer cannot be
given without knowing (at the very least) the language context.
See L<Unicode::Collate>, and I<Unicode Collation Algorithm>
http://www.unicode.org/unicode/reports/tr10/

=back

=head2 Miscellaneous

=over 4

=item *

Character Ranges and Classes

Character ranges in regular expression character classes (C</[a-z]/>)
and in the C<tr///> (also known as C<y///>) operator are not magically
Unicode-aware.  What this means that C<[A-Za-z]> will not magically start
to mean "all alphabetic letters" (not that it does mean that even for
8-bit characters, you should be using C</[[:alpha:]]/> for that).

For specifying things like that in regular expressions, you can use
the various Unicode properties, C<\pL> or perhaps C<\p{Alphabetic}>,
in this particular case.  You can use Unicode code points as the end
points of character ranges, but that means that particular code point
range, nothing more.  For further information (there are dozens
of Unicode character classes), see L<perlunicode>.

=item *

String-To-Number Conversions

Unicode does define several other decimal (and numeric) characters
than just the familiar 0 to 9, such as the Arabic and Indic digits.
Perl does not support string-to-number conversion for digits other
than ASCII 0 to 9 (and ASCII a to f for hexadecimal).

=back

=head2 Questions With Answers

=over 4

=item 

Will My Old Scripts Break?

Very probably not.  Unless you are generating Unicode characters
somehow, any old behaviour should be preserved.  About the only
behaviour that has changed and which could start generating Unicode
is the old behaviour of C<chr()> where supplying an argument more
than 255 produced a character modulo 255 (for example, C<chr(300)>
was equal to C<chr(45)>).

=item 

How Do I Make My Scripts Work With Unicode?

Very little work should be needed since nothing changes until you
somehow generate Unicode data.  The greatest trick will be getting
input as Unicode, and for that see the earlier I/O discussion.

=item 

How Do I Know Whether My String Is In Unicode?

You shouldn't care.  No, you really shouldn't.  If you have
to care (beyond the cases described above), it means that we
didn't get the transparency of Unicode quite right.

Okay, if you insist:

    use Encode 'is_utf8';
    print is_utf8($string) ? 1 : 0, "\n";

But note that this doesn't mean that any of the characters in the
string are necessary UTF-8 encoded, or that any of the characters have
code points greater than 0xFF (255) or even 0x80 (128), or that the
string has any characters at all.  All the C<is_utf8()> does is to
return the value of the internal "utf8ness" flag attached to the
$string.  If the flag is off, the bytes in the scalar are interpreted
as a single byte encoding.  If the flag is on, the bytes in the scalar
are interpreted as the (multibyte, variable-length) UTF-8 encoded code
points of the characters.  Bytes added to an UTF-8 encoded string are
automatically upgraded to UTF-8.  If mixed non-UTF8 and UTF-8 scalars
are merged (doublequoted interpolation, explicit concatenation, and
printf/sprintf parameter substitution), the result will be UTF-8 encoded
as if copies of the byte strings were upgraded to UTF-8: for example,

    $a = "ab\x80c";
    $b = "\x{100}";
    print "$a = $b\n";

the output string will be UTF-8-encoded "ab\x80c\x{100}\n", but note
that C<$a> will stay single byte encoded.

Sometimes you might really need to know the byte length of a string
instead of the character length.  For that use the C<bytes> pragma
and its only defined function C<length()>:

    my $unicode = chr(0x100);
    print length($unicode), "\n"; # will print 1
    use bytes;
    print length($unicode), "\n"; # will print 2 (the 0xC4 0x80 of the UTF-8)

=item 

How Do I Detect Data That's Not Valid In a Particular Encoding?

Use the C<Encode> package to try converting it.
For example,

    use Encode 'encode_utf8';
    if (encode_utf8($string_of_bytes_that_I_think_is_utf8)) {
        # valid
    } else {
        # invalid
    }

For UTF-8 only, you can use:

    use warnings;
    @chars = unpack("U0U*", $string_of_bytes_that_I_think_is_utf8);

If invalid, a C<Malformed UTF-8 character (byte 0x##) in
unpack> is produced. The "U0" means "expect strictly UTF-8
encoded Unicode".  Without that the C<unpack("U*", ...)>
would accept also data like C<chr(0xFF>), similarly to the
C<pack> as we saw earlier.

=item 

How Do I Convert Binary Data Into a Particular Encoding, Or Vice Versa?

This probably isn't as useful as you might think.
Normally, you shouldn't need to.

In one sense, what you are asking doesn't make much sense: Encodings
are for characters, and binary data is not "characters", so converting
"data" into some encoding isn't meaningful unless you know in what
character set and encoding the binary data is in, in which case it's
not binary data, now is it?

If you have a raw sequence of bytes that you know should be interpreted via
a particular encoding, you can use C<Encode>:

    use Encode 'from_to';
    from_to($data, "iso-8859-1", "utf-8"); # from latin-1 to utf-8

The call to from_to() changes the bytes in $data, but nothing material
about the nature of the string has changed as far as Perl is concerned.
Both before and after the call, the string $data contains just a bunch of
8-bit bytes. As far as Perl is concerned, the encoding of the string (as
Perl sees it) remains as "system-native 8-bit bytes".

You might relate this to a fictional 'Translate' module:

   use Translate;
   my $phrase = "Yes";
   Translate::from_to($phrase, 'english', 'deutsch');
   ## phrase now contains "Ja"

The contents of the string changes, but not the nature of the string.
Perl doesn't know any more after the call than before that the contents
of the string indicates the affirmative.

Back to converting data, if you have (or want) data in your system's
native 8-bit encoding (e.g. Latin-1, EBCDIC, etc.), you can use
pack/unpack to convert to/from Unicode.

    $native_string  = pack("C*", unpack("U*", $Unicode_string));
    $Unicode_string = pack("U*", unpack("C*", $native_string));

If you have a sequence of bytes you B<know> is valid UTF-8,
but Perl doesn't know it yet, you can make Perl a believer, too:

    use Encode 'decode_utf8';
    $Unicode = decode_utf8($bytes);

You can convert well-formed UTF-8 to a sequence of bytes, but if
you just want to convert random binary data into UTF-8, you can't.
Any random collection of bytes isn't well-formed UTF-8.  You can
use C<unpack("C*", $string)> for the former, and you can create
well-formed Unicode data by C<pack("U*", 0xff, ...)>.

=item 

How Do I Display Unicode?  How Do I Input Unicode?

See http://www.hclrss.demon.co.uk/unicode/ and
http://www.cl.cam.ac.uk/~mgk25/unicode.html

=item 

How Does Unicode Work With Traditional Locales?

In Perl, not very well.  Avoid using locales through the C<locale>
pragma.  Use only one or the other.

=back

=head2 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 C<0x> prefix means a hexadecimal number, the digits are 0-9 I<and>
a-f (or A-F, case doesn't matter).  Each hexadecimal digit represents
four bits, or half a byte.  C<print 0x..., "\n"> will show a
hexadecimal number in decimal, and C<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 C<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

=head2 Further Resources

=over 4

=item *

Unicode Consortium

    http://www.unicode.org/

=item *

Unicode FAQ

    http://www.unicode.org/unicode/faq/

=item *

Unicode Glossary

    http://www.unicode.org/glossary/

=item *

Unicode Useful Resources

    http://www.unicode.org/unicode/onlinedat/resources.html

=item *

Unicode and Multilingual Support in HTML, Fonts, Web Browsers and Other Applications

    http://www.hclrss.demon.co.uk/unicode/

=item *

UTF-8 and Unicode FAQ for Unix/Linux

    http://www.cl.cam.ac.uk/~mgk25/unicode.html

=item *

Legacy Character Sets

    http://www.czyborra.com/
    http://www.eki.ee/letter/

=item *

The Unicode support files live within the Perl installation in the
directory

    $Config{installprivlib}/unicore

in Perl 5.8.0 or newer, and 

    $Config{installprivlib}/unicode

in the Perl 5.6 series.  (The renaming to F<lib/unicore> was done to
avoid naming conflicts with lib/Unicode in case-insensitive filesystems.)
The main Unicode data file is F<UnicodeData.txt> (or F<Unicode.301> in
Perl 5.6.1.)  You can find the C<$Config{installprivlib}> by

    perl "-V:installprivlib"

Note that some of the files have been renamed from the Unicode
standard since the Perl installation tries to live by the "8.3"
filenaming restrictions.  The renamings are shown in the
accompanying F<rename> file.

You can explore various information from the Unicode data files using
the C<Unicode::UCD> module.

=back

=head1 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 C<Unicode::String>,
C<Unicode::Map8>, and C<Unicode::Map>, available from CPAN.
If you have the GNU recode installed, you can also use the
Perl frontend C<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;

=head1 SEE ALSO

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

=head1 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.

=head1 AUTHOR, COPYRIGHT, AND LICENSE

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

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