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Gurusamy Sarathy

NAME

perlunicode - Unicode support in Perl

DESCRIPTION

WARNING: The implementation of Unicode support in Perl is incomplete. Expect sudden and unannounced changes!

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 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. Later, in "Character encodings for input and output", we'll see how such inputs may be marked as being Unicode character data sources.

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 bytes.

The utf8 pragma is primarily a compatibility device that enables recognition of UTF-8 in literals encountered by the parser. It is also 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 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 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 is 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. In Perl v5.6, this is only enabled if the lexical scope has a use utf8 declaration (due to compatibility needs) but future versions may enable this by default.

    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}. A character in the Latin-1 range (128..255) should be written \x{ab} rather than \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 -w is turned on, it will produce a warning that you might be generating invalid UTF-8.

  • 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.)

    This also needs use utf8 currently. [XXX: Why?!? High-bit chars were syntax errors when they occurred within identifiers in previous versions, so this should probably be enabled by default.]

  • 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).)

    Unicode support in regular expressions needs use utf8 currently. [XXX: Because the SWASH routines need to be loaded. And the RE engine appears to need an overhaul to dynamically match Unicode anyway--the current RE compiler creates different nodes with and without use utf8.]

  • 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.

    use utf8 is needed to enable this. See above.

  • 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}.

    use utf8 is needed to enable this. See above.

  • 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*).

    use utf8 is needed to enable this. See above.

  • The 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, s/// doesn't take /U or /C (yet?).

    use utf8 is needed to enable this. See above.

  • 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.

  • 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

bytes, utf8, "${^WIDE_SYSTEM_CALLS}" in perlvar