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+=head1 NAME
+
+perlunicode - Unicode support in Perl
+
+=head1 DESCRIPTION
+
+=head2 Important Caveat
+
+WARNING: The implementation of Unicode support in Perl is incomplete.
+
+The following areas need further work.
+
+=over
+
+=item Input and Output Disciplines
+
+There is currently no easy way to mark data read from a file or other
+external source as being utf8.  This will be one of the major areas of
+focus in the near future.
+
+=item Regular Expressions
+
+The existing regular expression compiler does not produce polymorphic
+opcodes.  This means that the determination on whether to match Unicode
+characters is made when the pattern is compiled, based on whether the
+pattern contains Unicode characters, and not when the matching happens
+at run time.  This needs to be changed to adaptively match Unicode if
+the string to be matched is Unicode.
+
+=item C<use utf8> still needed to enable a few features
+
+The C<utf8> pragma implements the tables used for Unicode support.  These
+tables are automatically loaded on demand, so the C<utf8> pragma need not
+normally be used.
+
+However, as a compatibility measure, this pragma must be explicitly used
+to enable recognition of UTF-8 encoded literals and identifiers in the
+source text.
+
+=back
+
+=head2 Byte and Character semantics
+
+Beginning with version 5.6, Perl uses logically wide characters to
+represent strings internally.  This internal representation of strings
+uses the UTF-8 encoding.
+
+In future, Perl-level operations can be expected to work with characters
+rather than bytes, in general.
+
+However, as strictly an interim compatibility measure, Perl v5.6 aims to
+provide a safe migration path from byte semantics to character semantics
+for programs.  For operations where Perl can unambiguously decide that the
+input data is characters, Perl now switches to character semantics.
+For operations where this determination cannot be made without additional
+information from the user, Perl decides in favor of compatibility, and
+chooses to use byte semantics.
+
+This behavior preserves compatibility with earlier versions of Perl,
+which allowed byte semantics in Perl operations, but only as long as
+none of the program's inputs are marked as being as source of Unicode
+character data.  Such data may come from filehandles, from calls to
+external programs, from information provided by the system (such as %ENV),
+or from literals and constants in the source text.
+
+If the C<-C> command line switch is used, (or the ${^WIDE_SYSTEM_CALLS}
+global flag is set to C<1>), all system calls will use the
+corresponding wide character APIs.  This is currently only implemented
+on Windows.
+
+Regardless of the above, the C<bytes> pragma can always be used to force
+byte semantics in a particular lexical scope.  See L<bytes>.
+
+The C<utf8> pragma is primarily a compatibility device that enables
+recognition of UTF-8 in literals encountered by the parser.  It may also
+be used for enabling some of the more experimental Unicode support features.
+Note that this pragma is only required until a future version of Perl
+in which character semantics will become the default.  This pragma may
+then become a no-op.  See L<utf8>.
+
+Unless mentioned otherwise, Perl operators will use character semantics
+when they are dealing with Unicode data, and byte semantics otherwise.
+Thus, character semantics for these operations apply transparently; if
+the input data came from a Unicode source (for example, by adding a
+character encoding discipline to the filehandle whence it came, or a
+literal UTF-8 string constant in the program), character semantics
+apply; otherwise, byte semantics are in effect.  To force byte semantics
+on Unicode data, the C<bytes> pragma should be used.
+
+Under character semantics, many operations that formerly operated on
+bytes change to operating on characters.  For ASCII data this makes
+no difference, because UTF-8 stores ASCII in single bytes, but for
+any character greater than C<chr(127)>, the character may be stored in
+a sequence of two or more bytes, all of which have the high bit set.
+But by and large, the user need not worry about this, because Perl
+hides it from the user.  A character in Perl is logically just a number
+ranging from 0 to 2**32 or so.  Larger characters encode to longer
+sequences of bytes internally, but again, this is just an internal
+detail which is hidden at the Perl level.
+
+=head2 Effects of character semantics
+
+Character semantics have the following effects:
+
+=over 4
+
+=item *
+
+Strings and patterns may contain characters that have an ordinal value
+larger than 255.
+
+Presuming you use a Unicode editor to edit your program, such characters
+will typically occur directly within the literal strings as UTF-8
+characters, but you can also specify a particular character with an
+extension of the C<\x> notation.  UTF-8 characters are specified by
+putting the hexadecimal code within curlies after the C<\x>.  For instance,
+a Unicode smiley face is C<\x{263A}>.  A character in the Latin-1 range
+(128..255) should be written C<\x{ab}> rather than C<\xab>, since the
+former will turn into a two-byte UTF-8 code, while the latter will
+continue to be interpreted as generating a 8-bit byte rather than a
+character.  In fact, if the C<use warnings> pragma of the C<-w> switch
+is turned on, it will produce a warning
+that you might be generating invalid UTF-8.
+
+=item *
+
+Identifiers within the Perl script may contain Unicode alphanumeric
+characters, including ideographs.  (You are currently on your own when
+it comes to using the canonical forms of characters--Perl doesn't (yet)
+attempt to canonicalize variable names for you.)
+
+=item *
+
+Regular expressions match characters instead of bytes.  For instance,
+"." matches a character instead of a byte.  (However, the C<\C> pattern
+is provided to force a match a single byte ("C<char>" in C, hence
+C<\C>).)
+
+=item *
+
+Character classes in regular expressions match characters instead of
+bytes, and match against the character properties specified in the
+Unicode properties database.  So C<\w> can be used to match an ideograph,
+for instance.
+
+=item *
+
+Named Unicode properties and block ranges make be used as character
+classes via the new C<\p{}> (matches property) and C<\P{}> (doesn't
+match property) constructs.  For instance, C<\p{Lu}> matches any
+character with the Unicode uppercase property, while C<\p{M}> matches
+any mark character.  Single letter properties may omit the brackets, so
+that can be written C<\pM> also.  Many predefined character classes are
+available, such as C<\p{IsMirrored}> and  C<\p{InTibetan}>.
+
+=item *
+
+The special pattern C<\X> match matches any extended Unicode sequence
+(a "combining character sequence" in Standardese), where the first
+character is a base character and subsequent characters are mark
+characters that apply to the base character.  It is equivalent to
+C<(?:\PM\pM*)>.
+
+=item *
+
+The C<tr///> operator translates characters instead of bytes.  It can also
+be forced to translate between 8-bit codes and UTF-8.  For instance, if you
+know your input in Latin-1, you can say:
+
+    while (<>) {
+	tr/\0-\xff//CU;		# latin1 char to utf8
+	...
+    }
+
+Similarly you could translate your output with
+
+    tr/\0-\x{ff}//UC;		# utf8 to latin1 char
+
+No, C<s///> doesn't take /U or /C (yet?).
+
+=item *
+
+Case translation operators use the Unicode case translation tables
+when provided character input.  Note that C<uc()> translates to
+uppercase, while C<ucfirst> translates to titlecase (for languages
+that make the distinction).  Naturally the corresponding backslash
+sequences have the same semantics.
+
+=item *
+
+Most operators that deal with positions or lengths in the string will
+automatically switch to using character positions, including C<chop()>,
+C<substr()>, C<pos()>, C<index()>, C<rindex()>, C<sprintf()>,
+C<write()>, and C<length()>.  Operators that specifically don't switch
+include C<vec()>, C<pack()>, and C<unpack()>.  Operators that really
+don't care include C<chomp()>, as well as any other operator that
+treats a string as a bucket of bits, such as C<sort()>, and the
+operators dealing with filenames.
+
+=item *
+
+The C<pack()>/C<unpack()> letters "C<c>" and "C<C>" do I<not> change,
+since they're often used for byte-oriented formats.  (Again, think
+"C<char>" in the C language.)  However, there is a new "C<U>" specifier
+that will convert between UTF-8 characters and integers.  (It works
+outside of the utf8 pragma too.)
+
+=item *
+
+The C<chr()> and C<ord()> functions work on characters.  This is like
+C<pack("U")> and C<unpack("U")>, not like C<pack("C")> and
+C<unpack("C")>.  In fact, the latter are how you now emulate
+byte-oriented C<chr()> and C<ord()> under utf8.
+
+=item *
+
+And finally, C<scalar reverse()> reverses by character rather than by byte.
+
+=back
+
+=head2 Character encodings for input and output
+
+[XXX: This feature is not yet implemented.]
+
+=head1 CAVEATS
+
+As of yet, there is no method for automatically coercing input and
+output to some encoding other than UTF-8.  This is planned in the near
+future, however.
+
+Whether an arbitrary piece of data will be treated as "characters" or
+"bytes" by internal operations cannot be divined at the current time.
+
+Use of locales with utf8 may lead to odd results.  Currently there is
+some attempt to apply 8-bit locale info to characters in the range
+0..255, but this is demonstrably incorrect for locales that use
+characters above that range (when mapped into Unicode).  It will also
+tend to run slower.  Avoidance of locales is strongly encouraged.
+
+=head1 SEE ALSO
+
+L<bytes>, L<utf8>, L<perlvar/"${^WIDE_SYSTEM_CALLS}">
+
+=cut