=head1 DESCRIPTION
+=head2 Important Caveat
+
WARNING: The implementation of Unicode support in Perl is incomplete.
-Expect sudden and unannounced changes!
+
+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 will expect to work with characters
+In future, Perl-level operations can be expected to work with characters
rather than bytes, in general.
-However, Perl v5.6 aims to provide a safe migration path from byte
-semantics to character semantics for programs. To preserve compatibility
-with earlier versions of Perl which allowed byte semantics in Perl
-operations (owing to the fact that the internal representation for
-characters was in bytes) byte semantics will continue to be in effect
-until a the C<utf8> pragma is used in the C<main> package, or the C<$^U>
-global flag is explicitly set.
+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 is stored in
+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
sequences of bytes internally, but again, this is just an internal
detail which is hidden at the Perl level.
-The C<byte> pragma can be used to force byte semantics in a particular
-lexical scope. See L<byte>.
-
-The C<utf8> pragma is a compatibility device to enables recognition
-of UTF-8 in literals encountered by the parser. It is also used
-for enabling some 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>.
+=head2 Effects of character semantics
Character semantics have the following effects:
=item *
Strings and patterns may contain characters that have an ordinal value
-larger than 255. In Perl v5.6, this is only enabled if the lexical
-scope has a C<use utf8> declaration (due to compatibility needs) but
-future versions may enable this by default.
+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 C<-w> is turned on, it will produce a warning
-that you might be generating invalid UTF-8.
+a Unicode smiley face is C<\x{263A}>.
=item *
it comes to using the canonical forms of characters--Perl doesn't (yet)
attempt to canonicalize variable names for you.)
-This also needs C<use utf8> currently. [XXX: Why? High-bit chars were
-syntax errors when they occurred within identifiers in previous versions,
-so this should be enabled by default.]
-
=item *
Regular expressions match characters instead of bytes. For instance,
is provided to force a match a single byte ("C<char>" in C, hence
C<\C>).)
-Unicode support in regular expressions needs C<use utf8> currently.
-[XXX: Because the SWASH routines need to be loaded. And the RE engine
-appears to need an overhaul to Unicode by default anyway.]
-
=item *
Character classes in regular expressions match characters instead of
Unicode properties database. So C<\w> can be used to match an ideograph,
for instance.
-C<use utf8> is needed to enable this. See above.
-
=item *
Named Unicode properties and block ranges make be used as character
that can be written C<\pM> also. Many predefined character classes are
available, such as C<\p{IsMirrored}> and C<\p{InTibetan}>.
-C<use utf8> is needed to enable this. See above.
-
=item *
The special pattern C<\X> match matches any extended Unicode sequence
characters that apply to the base character. It is equivalent to
C<(?:\PM\pM*)>.
-C<use utf8> is needed to enable this. See above.
-
=item *
The C<tr///> operator translates characters instead of bytes. It can also
-be forced to translate between 8-bit codes and UTF-8 regardless of the
-surrounding utf8 state. For instance, if you know your input in Latin-1,
-you can say:
+be forced to translate between 8-bit codes and UTF-8. For instance, if you
+know your input in Latin-1, you can say:
- use utf8;
while (<>) {
tr/\0-\xff//CU; # latin1 char to utf8
...
No, C<s///> doesn't take /U or /C (yet?).
-C<use utf8> is needed to enable this. See above.
-
=item *
Case translation operators use the Unicode case translation tables
=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 a piece of data will be treated as "characters" or "bytes"
-by internal operations cannot be divined at the current time.
+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
=head1 SEE ALSO
-L<byte>, L<utf8>, L<perlvar/"$^U">
+L<bytes>, L<utf8>, L<perlvar/"${^WIDE_SYSTEM_CALLS}">
=cut
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