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PERLUNICODE(1)
NAME
perlunicode - Unicode support in Perl (EXPERIMENTAL, subject to change)
DESCRIPTION
Important Caveat
WARNING: As of the 5.6.1 release, the implementation of Unicode
support in Perl is incomplete, and continues to be highly experimental.
The following areas need further work. They are being rapidly addressed in
the 5.7.x development branch.
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.
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.
"use utf8" still needed to enable a few features
The "utf8" pragma implements the tables used for Unicode support.
These tables are automatically loaded on demand, so the "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.
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" command line switch is used, (or the ${^WIDE_SYSTEM_CALLS}
global flag is set to "1"), all system calls will use the corresponding
wide character APIs. This is currently only implemented on Windows.
Regardless of the above, the "bytes" pragma can always be used to force
byte semantics in a particular lexical scope. See the bytes manpage.
The "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 the utf8 manpage.
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
"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 "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.
Effects of character semantics
Character semantics have the following effects:
· 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 "\x" notation. UTF-8 characters are specified by
putting the hexadecimal code within curlies after the "\x". For
instance, a Unicode smiley face is "\x{263A}".
· 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.)
· Regular expressions match characters instead of bytes. For instance,
"." matches a character instead of a byte. (However, the "\C" pattern
is provided to force a match a single byte (""char"" in C, hence
"\C").)
· Character classes in regular expressions match characters instead of
bytes, and match against the character properties specified in the
Unicode properties database. So "\w" can be used to match an
ideograph, for instance.
· Named Unicode properties and block ranges make be used as character
classes via the new "\p{}" (matches property) and "\P{}" (doesn't match
property) constructs. For instance, "\p{Lu}" matches any character
with the Unicode uppercase property, while "\p{M}" matches any mark
character. Single letter properties may omit the brackets, so that can
be written "\pM" also. Many predefined character classes are
available, such as "\p{IsMirrored}" and "\p{InTibetan}".
· The special pattern "\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
"(?:\PM\pM*)".
· The "tr///" operator translates characters instead of bytes. Note that
the "tr///CU" functionality has been removed, as the interface was a
mistake. For similar functionality see pack('U0', ...) and pack('C0',
...).
· Case translation operators use the Unicode case translation tables when
provided character input. Note that "uc()" translates to uppercase,
while "ucfirst" translates to titlecase (for languages that make the
distinction). Naturally the corresponding backslash sequences have the
same semantics.
· Most operators that deal with positions or lengths in the string will
automatically switch to using character positions, including "chop()",
"substr()", "pos()", "index()", "rindex()", "sprintf()", "write()", and
"length()". Operators that specifically don't switch include "vec()",
"pack()", and "unpack()". Operators that really don't care include
"chomp()", as well as any other operator that treats a string as a
bucket of bits, such as "sort()", and the operators dealing with
filenames.
· The "pack()"/"unpack()" letters ""c"" and ""C"" do not change, since
they're often used for byte-oriented formats. (Again, think ""char""
in the C language.) However, there is a new ""U"" specifier that will
convert between UTF-8 characters and integers. (It works outside of
the utf8 pragma too.)
· The "chr()" and "ord()" functions work on characters. This is like
"pack("U")" and "unpack("U")", not like "pack("C")" and "unpack("C")".
In fact, the latter are how you now emulate byte-oriented "chr()" and
"ord()" under utf8.
· The bit string operators "& | ^ ~" 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
"~" (the bit complement) and characters both less than 256 and equal or
greater than 256. Most importantly, the DeMorgan's laws ("~($x|$y) eq
~$x&~$y", "~($x&$y) eq ~$x|~$y") won't hold. Another way to look at
this is that the complement cannot return both the 8-bit (byte) wide
bit complement, and the full character wide bit complement.
· And finally, "scalar reverse()" reverses by character rather than by
byte.
Character encodings for input and output
[XXX: This feature is not yet implemented.]
CAVEATS
As of yet, there is no method for automatically coercing input and output
to some encoding other than UTF-8. This is planned in the near future,
however.
Whether an arbitrary piece of data will be treated as "characters" or
"bytes" by internal operations cannot be divined at the current time.
Use of locales with utf8 may lead to odd results. Currently there is some
attempt to apply 8-bit locale info to characters in the range 0..255, but
this is demonstrably incorrect for locales that use characters above that
range (when mapped into Unicode). It will also tend to run slower.
Avoidance of locales is strongly encouraged.
SEE ALSO
the bytes manpage, the utf8 manpage, the section on "${^WIDE_SYSTEM_CALLS}"
in the perlvar manpage
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