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PERLUNIFAQ(1)          Perl Programmers Reference Guide          PERLUNIFAQ(1)



NAME
       perlunifaq - Perl Unicode FAQ

Q and A
       This is a list of questions and answers about Unicode in Perl, intended to be read after perlunitut.

   perlunitut isn't really a Unicode tutorial, is it?
       No, and this isn't really a Unicode FAQ.

       Perl has an abstracted interface for all supported character encodings, so they is actually a generic "Encode"
       tutorial and "Encode" FAQ. But many people think that Unicode is special and magical, and I didn't want to
       disappoint them, so I decided to call the document a Unicode tutorial.

   What character encodings does Perl support?
       To find out which character encodings your Perl supports, run:

           perl -MEncode -le "print for Encode->encodings(':all')"

   Which version of perl should I use?
       Well, if you can, upgrade to the most recent, but certainly 5.8.1 or newer.  The tutorial and FAQ are based on
       the status quo as of 5.8.8.

       You should also check your modules, and upgrade them if necessary. For example, HTML::Entities requires version
       >= 1.32 to function correctly, even though the changelog is silent about this.

   What about binary data, like images?
       Well, apart from a bare "binmode $fh", you shouldn't treat them specially.  (The binmode is needed because
       otherwise Perl may convert line endings on Win32 systems.)

       Be careful, though, to never combine text strings with binary strings. If you need text in a binary stream,
       encode your text strings first using the appropriate encoding, then join them with binary strings. See also:
       "What if I don't encode?".

   When should I decode or encode?
       Whenever you're communicating text with anything that is external to your perl process, like a database, a text
       file, a socket, or another program. Even if the thing you're communicating with is also written in Perl.

   What if I don't decode?
       Whenever your encoded, binary string is used together with a text string, Perl will assume that your binary
       string was encoded with ISO-8859-1, also known as latin-1. If it wasn't latin-1, then your data is unpleasantly
       converted. For example, if it was UTF-8, the individual bytes of multibyte characters are seen as separate
       characters, and then again converted to UTF-8. Such double encoding can be compared to double HTML encoding
       (">"), or double URI encoding (%253E).

       This silent implicit decoding is known as "upgrading". That may sound positive, but it's best to avoid it.

   What if I don't encode?
       Your text string will be sent using the bytes in Perl's internal format. In some cases, Perl will warn you that
       you're doing something wrong, with a friendly warning:

           Wide character in print at example.pl line 2.

       Because the internal format is often UTF-8, these bugs are hard to spot, because UTF-8 is usually the encoding
       you wanted! But don't be lazy, and don't use the fact that Perl's internal format is UTF-8 to your advantage.
       Encode explicitly to avoid weird bugs, and to show to maintenance programmers that you thought this through.

   Is there a way to automatically decode or encode?
       If all data that comes from a certain handle is encoded in exactly the same way, you can tell the PerlIO system
       to automatically decode everything, with the "encoding" layer. If you do this, you can't accidentally forget to
       decode or encode anymore, on things that use the layered handle.

       You can provide this layer when "open"ing the file:

           open my $fh, '>:encoding(UTF-8)', $filename;  # auto encoding on write
           open my $fh, '<:encoding(UTF-8)', $filename;  # auto decoding on read

       Or if you already have an open filehandle:

           binmode $fh, ':encoding(UTF-8)';

       Some database drivers for DBI can also automatically encode and decode, but that is sometimes limited to the
       UTF-8 encoding.

   What if I don't know which encoding was used?
       Do whatever you can to find out, and if you have to: guess. (Don't forget to document your guess with a
       comment.)

       You could open the document in a web browser, and change the character set or character encoding until you can
       visually confirm that all characters look the way they should.

       There is no way to reliably detect the encoding automatically, so if people keep sending you data without
       charset indication, you may have to educate them.

   Can I use Unicode in my Perl sources?
       Yes, you can! If your sources are UTF-8 encoded, you can indicate that with the "use utf8" pragma.

           use utf8;

       This doesn't do anything to your input, or to your output. It only influences the way your sources are read.
       You can use Unicode in string literals, in identifiers (but they still have to be "word characters" according
       to "\w"), and even in custom delimiters.

   Data::Dumper doesn't restore the UTF8 flag; is it broken?
       No, Data::Dumper's Unicode abilities are as they should be. There have been some complaints that it should
       restore the UTF8 flag when the data is read again with "eval". However, you should really not look at the flag,
       and nothing indicates that Data::Dumper should break this rule.

       Here's what happens: when Perl reads in a string literal, it sticks to 8 bit encoding as long as it can. (But
       perhaps originally it was internally encoded as UTF-8, when you dumped it.) When it has to give that up because
       other characters are added to the text string, it silently upgrades the string to UTF-8.

       If you properly encode your strings for output, none of this is of your concern, and you can just "eval" dumped
       data as always.

   Why do regex character classes sometimes match only in the ASCII range?
   Why do some characters not uppercase or lowercase correctly?
       It seemed like a good idea at the time, to keep the semantics the same for standard strings, when Perl got
       Unicode support. While it might be repaired in the future, we now have to deal with the fact that Perl treats
       equal strings differently, depending on the internal state.

       Affected are "uc", "lc", "ucfirst", "lcfirst", "\U", "\L", "\u", "\l", "\d", "\s", "\w", "\D", "\S", "\W",
       "/.../i", "(?i:...)", "/[[:posix:]]/", and "quotemeta" (though this last should not cause any real problems).

       To force Unicode semantics, you can upgrade the internal representation to by doing "utf8::upgrade($string)".
       This can be used safely on any string, as it checks and does not change strings that have already been
       upgraded.

       For a more detailed discussion, see Unicode::Semantics on CPAN.

   How can I determine if a string is a text string or a binary string?
       You can't. Some use the UTF8 flag for this, but that's misuse, and makes well behaved modules like Data::Dumper
       look bad. The flag is useless for this purpose, because it's off when an 8 bit encoding (by default ISO-8859-1)
       is used to store the string.

       This is something you, the programmer, has to keep track of; sorry. You could consider adopting a kind of
       "Hungarian notation" to help with this.

   How do I convert from encoding FOO to encoding BAR?
       By first converting the FOO-encoded byte string to a text string, and then the text string to a BAR-encoded
       byte string:

           my $text_string = decode('FOO', $foo_string);
           my $bar_string  = encode('BAR', $text_string);

       or by skipping the text string part, and going directly from one binary encoding to the other:

           use Encode qw(from_to);
           from_to($string, 'FOO', 'BAR');  # changes contents of $string

       or by letting automatic decoding and encoding do all the work:

           open my $foofh, '<:encoding(FOO)', 'example.foo.txt';
           open my $barfh, '>:encoding(BAR)', 'example.bar.txt';
           print { $barfh } $_ while <$foofh>;

   What are "decode_utf8" and "encode_utf8"?
       These are alternate syntaxes for "decode('utf8', ...)" and "encode('utf8', ...)".

   What is a "wide character"?
       This is a term used both for characters with an ordinal value greater than 127, characters with an ordinal
       value greater than 255, or any character occupying than one byte, depending on the context.

       The Perl warning "Wide character in ..." is caused by a character with an ordinal value greater than 255. With
       no specified encoding layer, Perl tries to fit things in ISO-8859-1 for backward compatibility reasons. When it
       can't, it emits this warning (if warnings are enabled), and outputs UTF-8 encoded data instead.

       To avoid this warning and to avoid having different output encodings in a single stream, always specify an
       encoding explicitly, for example with a PerlIO layer:

           binmode STDOUT, ":encoding(UTF-8)";

INTERNALS
   What is "the UTF8 flag"?
       Please, unless you're hacking the internals, or debugging weirdness, don't think about the UTF8 flag at all.
       That means that you very probably shouldn't use "is_utf8", "_utf8_on" or "_utf8_off" at all.

       The UTF8 flag, also called SvUTF8, is an internal flag that indicates that the current internal representation
       is UTF-8. Without the flag, it is assumed to be ISO-8859-1. Perl converts between these automatically.

       One of Perl's internal formats happens to be UTF-8. Unfortunately, Perl can't keep a secret, so everyone knows
       about this. That is the source of much confusion. It's better to pretend that the internal format is some
       unknown encoding, and that you always have to encode and decode explicitly.

   What about the "use bytes" pragma?
       Don't use it. It makes no sense to deal with bytes in a text string, and it makes no sense to deal with
       characters in a byte string. Do the proper conversions (by decoding/encoding), and things will work out well:
       you get character counts for decoded data, and byte counts for encoded data.

       "use bytes" is usually a failed attempt to do something useful. Just forget about it.

   What about the "use encoding" pragma?
       Don't use it. Unfortunately, it assumes that the programmer's environment and that of the user will use the
       same encoding. It will use the same encoding for the source code and for STDIN and STDOUT. When a program is
       copied to another machine, the source code does not change, but the STDIO environment might.

       If you need non-ASCII characters in your source code, make it a UTF-8 encoded file and "use utf8".

       If you need to set the encoding for STDIN, STDOUT, and STDERR, for example based on the user's locale, "use
       open".

   What is the difference between ":encoding" and ":utf8"?
       Because UTF-8 is one of Perl's internal formats, you can often just skip the encoding or decoding step, and
       manipulate the UTF8 flag directly.

       Instead of ":encoding(UTF-8)", you can simply use ":utf8", which skips the encoding step if the data was
       already represented as UTF8 internally. This is widely accepted as good behavior when you're writing, but it
       can be dangerous when reading, because it causes internal inconsistency when you have invalid byte sequences.
       Using ":utf8" for input can sometimes result in security breaches, so please use ":encoding(UTF-8)" instead.

       Instead of "decode" and "encode", you could use "_utf8_on" and "_utf8_off", but this is considered bad style.
       Especially "_utf8_on" can be dangerous, for the same reason that ":utf8" can.

       There are some shortcuts for oneliners; see "-C" in perlrun.

   What's the difference between "UTF-8" and "utf8"?
       "UTF-8" is the official standard. "utf8" is Perl's way of being liberal in what it accepts. If you have to
       communicate with things that aren't so liberal, you may want to consider using "UTF-8". If you have to
       communicate with things that are too liberal, you may have to use "utf8". The full explanation is in Encode.

       "UTF-8" is internally known as "utf-8-strict". The tutorial uses UTF-8 consistently, even where utf8 is
       actually used internally, because the distinction can be hard to make, and is mostly irrelevant.

       For example, utf8 can be used for code points that don't exist in Unicode, like 9999999, but if you encode that
       to UTF-8, you get a substitution character (by default; see "Handling Malformed Data" in Encode for more ways
       of dealing with this.)

       Okay, if you insist: the "internal format" is utf8, not UTF-8. (When it's not some other encoding.)

   I lost track; what encoding is the internal format really?
       It's good that you lost track, because you shouldn't depend on the internal format being any specific encoding.
       But since you asked: by default, the internal format is either ISO-8859-1 (latin-1), or utf8, depending on the
       history of the string. On EBCDIC platforms, this may be different even.

       Perl knows how it stored the string internally, and will use that knowledge when you "encode". In other words:
       don't try to find out what the internal encoding for a certain string is, but instead just encode it into the
       encoding that you want.

AUTHOR
       Juerd Waalboer <#####@juerd.nl>

SEE ALSO
       perlunicode, perluniintro, Encode



perl v5.10.1                      2009-02-25                     PERLUNIFAQ(1)