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 perlunicode - Unicode support in Perl


perlunicode - Unicode support in Perl (EXPERIMENTAL, subject to change)


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:

Character encodings for input and output

[XXX: This feature is not yet implemented.]


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.


the bytes manpage, the utf8 manpage, ${^WIDE_SYSTEM_CALLS} in the perlvar manpage

 perlunicode - Unicode support in Perl