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perlunifaq (1)


perlunifaq - Perl Unicode FAQ


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Perl Programmers Reference Guide                    PERLUNIFAQ(1)

     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 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,

         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

     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

  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

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     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

     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 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

  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.

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     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

  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.  The plan is to fix this in the future, and
     the casing component has in fact mostly been fixed, but we
     have to deal with the fact that Perl treats equal strings
     differently, depending on the internal state.

     First the casing.  Just put a "use feature
     'unicode_strings'" near the beginning of your program.
     Within its lexical scope, "uc", "lc", "ucfirst", "lcfirst",
     and the regular expression escapes "\U", "\L", "\u", "\l"
     use Unicode semantics for changing case regardless of
     whether the UTF8 flag is on or not.  However, if you pass
     strings to subroutines in modules outside the pragma's

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     scope, they currently likely won't behave this way, and you
     have to try one of the solutions below.  There is another
     exception as well:  if you have furnished your own casing
     functions to override the default, these will not be called
     unless the UTF8 flag is on)

     This remains a problem for the regular expression constructs
     "\d", "\s", "\w", "\D", "\S", "\W", "/.../i", "(?i:...)",
     and "/[[:posix:]]/".

     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

  How can I determine if a string is a text string or a binary
     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

         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

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

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  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 more 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)";

  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

  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.

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     "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.

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     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

     Juerd Waalboer <>

     See attributes(5) for descriptions of the following

     |Availability   | runtime/perl-512 |
     |Stability      | Uncommitted      |
     perlunicode, perluniintro, Encode

     This software was built from source available at  The original
     community source was downloaded from

     Further information about this software can be found on the
     open source community website at

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