perlunifaq


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 this 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 assume the latest release.

       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.
       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

       The rationale for requiring this is to not break older programs that
       rely on the way things worked before Unicode came along.  Those older
       programs knew only about the ASCII character set, and so may not work
       properly for additional characters.  When a string is encoded in UTF-8,
       Perl assumes that the program is prepared to deal with Unicode, but
       when the string isn't, Perl assumes that only ASCII (unless it is an
       EBCDIC platform) is wanted, and so those characters that are not ASCII
       characters aren't recognized as to what they would be in Unicode.  "use
       feature 'unicode_strings'" tells Perl to treat all characters as
       Unicode, whether the string is encoded in UTF-8 or not, thus avoiding
       the problem.

       However, on earlier Perls, or if you pass strings to subroutines
       outside the feature's scope, you can force Unicode semantics by
       changing the encoding to UTF-8 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', ...)".

       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.
       (Actually Perl usually assumes the representation is ASCII; see "Why do
       regex character classes sometimes match only in the ASCII range?"
       above.)

       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

   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 v5.14.2                      2011-09-26                     PERLUNIFAQ(1)
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