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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 generally 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 I<logical> "character" can actually consist of more than one internal
+I<actual> "character" or code point. For Western languages, this is adequately
+modelled by 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>. Some non-western languages require more complicated
+models, so Unicode created the I<grapheme cluster> concept, and then the
+I<extended grapheme cluster>. For example, a Korean Hangul syllable is
+considered a single logical character, but most often consists of three actual
+Unicode characters: a leading consonant followed by an interior vowel followed
+by a trailing consonant.
+
+Whether to call these extended grapheme clusters "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.
+
+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 is "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 about Unicode blocks--that they 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 more information about
+scripts, see L<perlunicode/Scripts>.
+
+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. 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 needed only in much more restricted circumstances.> 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 (mostly) transparently upgraded
+to Unicode. There are some problems--see L<perlunicode/The "Unicode Bug">.
+
+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.
+
+Later Perl releases have added code that will not work on EBCDIC platforms, and
+no one has complained, so the divergence has continued. If you want to run
+Perl on an EBCDIC platform, send email to perlbug@perl.org
+
+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 grapheme clusters 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 an extended grapheme cluster.
+
+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
+well as what characters happen to be in the string at the time. If
+any of the characters are at code points C<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, ">:utf8", "file";
+
+ open FH, ">:encoding(ucs2)", "file";
+ open FH, ">:encoding(UTF-8)", "file";
+ open FH, ">:encoding(shift_jis)", "file";
+
+and on already open streams, use C<binmode()>:
+
+ 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 C<:utf8> layer
+must always be specified exactly like that; it is I<not> subject to
+the loose matching of encoding names. Also note that C<:utf8> is unsafe for
+input, because it accepts the data without validating that it is indeed valid
+UTF8.
+
+See L<PerlIO> for the C<:utf8> layer, L<PerlIO::encoding> and
+L<Encode::PerlIO> for the C<:encoding()> layer, and
+L<Encode::Supported> for many encodings supported by the C<Encode>
+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,'<:encoding(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
+the C<open> pragma. See L<open>, or look at the following example.
+
+ use open ':encoding(utf8)'; # input/output default encoding 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> layer
+
+ BEGIN { $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;
+
+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 layers. If you want to affect only certain
+streams, use explicit layers 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 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 the C<open()> and by the C<open>
+pragma allows for flexible names: 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::Supported>.
+
+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 layer,
+it is easy to mistakenly write code that keeps on expanding a file
+by repeatedly encoding the data:
+
+ # BAD CODE WARNING
+ open F, "file";
+ local $/; ## read in the whole file of 8-bit characters
+ $t = <F>;
+ close F;
+ open F, ">:encoding(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 ':encoding(utf8)'> would have avoided the
+bug, or explicitly opening also the F<file> for input as UTF-8.
+
+B<NOTE>: the C<:utf8> and C<:encoding> features work only if your
+Perl has been built with the new PerlIO feature (which is the default
+on most systems).
+
+=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 C<\x{...}>, control characters (like C<\n>) are
+displayed as C<\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("W*", $_[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.
+
+=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 C<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 layers). 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 of whatever the string
+encoding happens to be, or C<unpack("U0..", ...)> to get the bytes of the
+UTF-8 encoding:
+
+ # this prints c4 80 for the UTF-8 bytes 0xc4 0x80
+ print join(" ", unpack("U0(H2)*", 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 C<UTF8> flag in FLAGS and both the UTF-8 bytes
+and Unicode characters in C<PV>. See also later in this document
+the discussion about the C<utf8::is_utf8()> function.
+
+=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 should be considered equal, or even As of any case.
+
+The long answer is that you need to consider character normalization
+and casing issues: see L<Unicode::Normalize>, Unicode Technical Report #15,
+L<Unicode Normalization Forms|http://www.unicode.org/unicode/reports/tr15> and
+sections on case mapping in the L<Unicode Standard|http://www.unicode.org>.
+
+As of Perl 5.8.0, the "Full" case-folding of I<Case
+Mappings/SpecialCasing> is implemented, but bugs remain in C<qr//i> with them.
+
+=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
+C<0x00C1> > C<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>
+L<http://www.unicode.org/unicode/reports/tr10/>
+
+=back
+
+=head2 Miscellaneous
+
+=over 4
+
+=item *
+
+Character Ranges and Classes
+
+Character ranges in regular expression bracketed character classes ( e.g.,
+C</[a-z]/>) and in the C<tr///> (also known as C<y///>) operator are not
+magically Unicode-aware. What this means is 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; for those, if you are using locales (L<perllocale>),
+use C</[[:alpha:]]/>; and if not, use the 8-bit-aware property C<\p{alpha}>).
+
+All the properties that begin with C<\p> (and its inverse C<\P>) are actually
+character classes that are Unicode-aware. There are dozens of them, see
+L<perluniprops>.
+
+You can use Unicode code points as the end points of character ranges, and the
+range will include all Unicode code points that lie between those end points.
+
+=item *
+
+String-To-Number Conversions
+
+Unicode does define several other decimal--and numeric--characters
+besides 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, 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. C<chr(300)>, for example, was equal
+to C<chr(45)> or "-" (in ASCII), now it is LATIN CAPITAL LETTER I WITH
+BREVE.
+
+=item *
+
+How Do I Make My Scripts Work With Unicode?
+
+Very little work should be needed since nothing changes until you
+generate Unicode data. The most important thing is getting input as
+Unicode; for that, see the earlier I/O discussion.
+
+=item *
+
+How Do I Know Whether My String Is In Unicode?
+
+You shouldn't have to care. But you may, because currently the semantics of the
+characters whose ordinals are in the range 128 to 255 are different depending on
+whether the string they are contained within is in Unicode or not.
+(See L<perlunicode/When Unicode Does Not Happen>.)
+
+To determine if a string is in Unicode, use:
+
+ print utf8::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
+C<$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 (variable-length, potentially multi-byte) UTF-8 encoded
+code points of the characters. Bytes added to a UTF-8 encoded string are
+automatically upgraded to UTF-8. If mixed non-UTF-8 and UTF-8 scalars
+are merged (double-quoted 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 = $b\n";
+
+the output string will be UTF-8-encoded C<ab\x80c = \x{100}\n>, but
+C<$a> will stay byte-encoded.
+
+Sometimes you might really need to know the byte length of a string
+instead of the character length. For that use either the
+C<Encode::encode_utf8()> function or the C<bytes> pragma and
+the C<length()> function:
+
+ my $unicode = chr(0x100);
+ print length($unicode), "\n"; # will print 1
+ require Encode;
+ print length(Encode::encode_utf8($unicode)), "\n"; # will print 2
+ use bytes;
+ print length($unicode), "\n"; # will also print 2
+ # (the 0xC4 0x80 of the UTF-8)
+ no bytes;
+
+=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 'decode_utf8';
+
+ if (eval { decode_utf8($string, Encode::FB_CROAK); 1 }) {
+ # $string is valid utf8
+ } else {
+ # $string is not valid utf8
+ }
+
+Or use C<unpack> to try decoding it:
+
+ use warnings;
+ @chars = unpack("C0U*", $string_of_bytes_that_I_think_is_utf8);
+
+If invalid, a C<Malformed UTF-8 character> warning is produced. The "C0" means
+"process the string character per character". Without that, the
+C<unpack("U*", ...)> would work in C<U0> mode (the default if the format
+string starts with C<U>) and it would return the bytes making up the UTF-8
+encoding of the target string, something that will always work.
+
+=item *
+
+How Do I Convert Binary Data Into a Particular Encoding, Or Vice Versa?
+
+This probably isn't as useful as you might think.
+Normally, you shouldn't need to.
+
+In one sense, what you are asking doesn't make much sense: encodings
+are for characters, and binary data are not "characters", so converting
+"data" into some encoding isn't meaningful unless you know in what
+character set and encoding the binary data is in, in which case it's
+not just binary data, now is it?
+
+If you have a raw sequence of bytes that you know should be
+interpreted via a particular encoding, you can use C<Encode>:
+
+ use Encode 'from_to';
+ from_to($data, "iso-8859-1", "utf-8"); # from latin-1 to utf-8
+
+The call to C<from_to()> changes the bytes in C<$data>, but nothing
+material about the nature of the string has changed as far as Perl is
+concerned. Both before and after the call, the string C<$data>
+contains just a bunch of 8-bit bytes. As far as Perl is concerned,
+the encoding of the string 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("W*", unpack("U*", $Unicode_string));
+ $Unicode_string = pack("U*", unpack("W*", $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);
+
+or:
+
+ $Unicode = pack("U0a*", $bytes);
+
+You can find the bytes that make up a UTF-8 sequence with
+
+ @bytes = unpack("C*", $Unicode_string)
+
+and you can create well-formed Unicode with
+
+ $Unicode_string = pack("U*", 0xff, ...)
+
+=item *
+
+How Do I Display Unicode? How Do I Input Unicode?
+
+See L<http://www.alanwood.net/unicode/> and
+L<http://www.cl.cam.ac.uk/~mgk25/unicode.html>
+
+=item *
+
+How Does Unicode Work With Traditional Locales?
+
+In Perl, not very well. Avoid using locales through the C<locale>
+pragma. Use only one or the other. But see L<perlrun> for the
+description of the C<-C> switch and its environment counterpart,
+C<$ENV{PERL_UNICODE}> to see how to enable various Unicode features,
+for example by using locale settings.
+
+=back
+
+=head2 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. The C<U+HHHH> notation uses hexadecimal,
+for example.
+
+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
+"hex digits" 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
+
+L<http://www.unicode.org/>
+
+=item *
+
+Unicode FAQ
+
+L<http://www.unicode.org/unicode/faq/>
+
+=item *
+
+Unicode Glossary
+
+L<http://www.unicode.org/glossary/>
+
+=item *
+
+Unicode Useful Resources
+
+L<http://www.unicode.org/unicode/onlinedat/resources.html>
+
+=item *
+
+Unicode and Multilingual Support in HTML, Fonts, Web Browsers and Other Applications
+
+L<http://www.alanwood.net/unicode/>
+
+=item *
+
+UTF-8 and Unicode FAQ for Unix/Linux
+
+L<http://www.cl.cam.ac.uk/~mgk25/unicode.html>
+
+=item *
+
+Legacy Character Sets
+
+L<http://www.czyborra.com/>
+L<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"
+
+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 front-end C<Convert::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-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<perlunitut>, L<perlunicode>, L<Encode>, L<open>, L<utf8>, L<bytes>,
+L<perlretut>, L<perlrun>, L<Unicode::Collate>, L<Unicode::Normalize>,
+L<Unicode::UCD>
+
+=head1 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.
+
+=head1 AUTHOR, COPYRIGHT, AND LICENSE
+
+Copyright 2001-2002 Jarkko Hietaniemi E<lt>jhi@iki.fiE<gt>
+
+This document may be distributed under the same terms as Perl itself.