=head1 DESCRIPTION
-WARNING: The implementation of Unicode support in Perl is incomplete.
-Expect sudden and unannounced changes!
+=head2 Important Caveats
+
+WARNING: While the implementation of Unicode support in Perl is now fairly
+complete it is still evolving to some extent.
+
+In particular the way Unicode is handled on EBCDIC platforms is still rather
+experimental. On such a platform references to UTF-8 encoding in this
+document and elsewhere should be read as meaning UTF-EBCDIC as specified
+in Unicode Technical Report 16 unless ASCII vs EBCDIC issues are specifically
+discussed. There is no C<utfebcdic> pragma or ":utfebcdic" layer, rather
+"utf8" and ":utf8" are re-used to mean platform's "natural" 8-bit encoding
+of Unicode. See L<perlebcdic> for more discussion of the issues.
+
+The following areas are still under development.
+
+=over 4
+
+=item Input and Output Disciplines
+
+A filehandle can be marked as containing perl's internal Unicode encoding
+(UTF-8 or UTF-EBCDIC) by opening it with the ":utf8" layer.
+Other encodings can be converted to perl's encoding on input, or from
+perl's encoding on output by use of the ":encoding()" layer.
+There is not yet a clean way to mark the perl source itself as being
+in an particular encoding.
+
+=item Regular Expressions
+
+The regular expression compiler does now attempt to produce
+polymorphic opcodes. That is the pattern should now adapt to the data
+and automatically switch to the Unicode character scheme when presented
+with Unicode data, or a traditional byte scheme when presented with
+byte data. The implementation is still new and (particularly on
+EBCDIC platforms) may need further work.
+
+=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 on ASCII based machines or recognize UTF-EBCDIC encoded literals
+and identifiers on EBCDIC based machines.
+
+=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.
+uses either the UTF-8 or the UTF-EBCDIC 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 since UNIXes lack API standard on this area.
+
+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|EBCDIC) 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.
+For C1 controls or Latin 1 characters on an EBCDIC platform the character
+may be stored in a UTF-EBCDIC multi byte sequence.
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.
-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
+will typically occur directly within the literal strings as UTF-(8|EBCDIC)
characters, but you can also specify a particular character with an
-extension of the C<\x> notation. UTF-8 characters are specified by
+extension of the C<\x> notation. UTF-X 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
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}>.
-
-C<use utf8> is needed to enable this. See above.
+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}>. The
+names of the C<In> classes are the official Unicode block names but
+with all non-alphanumeric characters removed, for example the block
+name C<"Latin-1 Supplement"> becomes C<\p{InLatin1Supplement}>.
=item *
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:
-
- use utf8;
- 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?).
-
-C<use utf8> is needed to enable this. See above.
+The C<tr///> operator translates characters instead of bytes. Note
+that the C<tr///CU> functionality has been removed, as the interface
+was a mistake. For similar functionality see pack('U0', ...) and
+pack('C0', ...).
=item *
=item *
+The bit string operators C<& | ^ ~> can operate on character data.
+However, for backward compatibility reasons (bit string operations
+when the characters all are less than 256 in ordinal value) one cannot
+mix C<~> (the bit complement) and characters both less than 256 and
+equal or greater than 256. Most importantly, the DeMorgan's laws
+(C<~($x|$y) eq ~$x&~$y>, C<~($x&$y) eq ~$x|~$y>) won't hold.
+Another way to look at this is that the complement cannot return
+B<both> the 8-bit (byte) wide bit complement, and the full character
+wide bit complement.
+
+=item *
+
And finally, C<scalar reverse()> reverses by character rather than by byte.
=back
+=head2 Character encodings for input and output
+
+See L<Encode>.
+
=head1 CAVEATS
As of yet, there is no method for automatically coercing input and
-output to some encoding other than UTF-8. This is planned in the near
-future, however.
+output to some encoding other than UTF-8 or UTF-EBCDIC. This is planned
+in the near future, however.
-Whether a piece of data will be treated as "characters" or "bytes"
-by internal operations cannot be divined at the current time.
+Whether an arbitrary piece of data will be treated as "characters" or
+"bytes" by internal operations cannot be divined at the current time.
Use of locales with utf8 may lead to odd results. Currently there is
some attempt to apply 8-bit locale info to characters in the range
=head1 SEE ALSO
-L<byte>, L<utf8>, L<perlvar/"$^U">
+L<bytes>, L<utf8>, L<perlvar/"${^WIDE_SYSTEM_CALLS}">
=cut
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