Perl has an abstracted interface for all supported character encodings, so thisis actually a generic Encode tutorial and Encode FAQ. But many peoplethink that Unicode is special and magical, and I didn't want to disappointthem, 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 thechangelog 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 Win32systems.)

Be careful, though, to never combine text strings with binary strings. If youneed text in a binary stream, encode your text strings first using theappropriate encoding, then join them with binary strings. See also: "What if Idon't encode?".

When should I decode or encode?

Whenever you're communicating text with anything that is external to your perlprocess, like a database, a text file, a socket, or another program. Even ifthe 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, Perlwill assume that your binary string was encoded with ISO-8859-1, also known aslatin-1. If it wasn't latin-1, then your data is unpleasantly converted. Forexample, if it was UTF-8, the individual bytes of multibyte characters are seenas separate characters, and then again converted to UTF-8. Such double encodingcan be compared to double HTML encoding (&gt;), or double URI encoding(%253E).

This silent implicit decoding is known as "upgrading". That may soundpositive, 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. Insome cases, Perl will warn you that you're doing something wrong, with afriendly 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'tuse the fact that Perl's internal format is UTF-8 to your advantage. Encodeexplicitly to avoid weird bugs, and to show to maintenance programmers that youthought 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 sameway, you can tell the PerlIO system to automatically decode everything, withthe encoding layer. If you do this, you can't accidentally forget to decodeor encode anymore, on things that use the layered handle.

You can provide this layer when opening 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, butthat 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 todocument your guess with a comment.)

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

There is no way to reliably detect the encoding automatically, so if peoplekeep 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 theuse utf8 pragma.

 use utf8;

This doesn't do anything to your input, or to your output. It only influencesthe way your sources are read. You can use Unicode in string literals, inidentifiers (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 beensome complaints that it should restore the UTF8 flag when the data is readagain with eval. However, you should really not look at the flag, andnothing indicates that Data::Dumper should break this rule.

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

If you properly encode your strings for output, none of this is of yourconcern, 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?

Starting in Perl 5.14 (and partially in Perl 5.12), just put ause feature 'unicode_strings' near the beginning of your program.Within its lexical scope you shouldn't have this problem. It also isautomatically enabled under use feature ':5.12' or using -E on thecommand line for Perl 5.12 or higher.

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

However, on earlier Perls, or if you pass strings to subroutines outsidethe feature's scope, you can force Unicode semantics by changing theencoding to UTF-8 by doing utf8::upgrade($string). This can be usedsafely on any string, as it checks and does not change strings that havealready 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 wellbehaved modules like Data::Dumper look bad. The flag is useless for thispurpose, because it's off when an 8 bit encoding (by default ISO-8859-1) isused to store the string.

This is something you, the programmer, has to keep track of; sorry. You couldconsider 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 thetext 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 binaryencoding 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 occupyingmore than one byte, depending on the context.

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

To avoid this warning and to avoid having different output encodings in a singlestream, 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'tthink about the UTF8 flag at all. That means that you very probably shouldn'tuse is_utf8, _utf8_on or _utf8_off at all.

The UTF8 flag, also called SvUTF8, is an internal flag that indicates that thecurrent internal representation is UTF-8. Without the flag, it is assumed to beISO-8859-1. Perl converts between these automatically. (Actually Perl usuallyassumes 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'tkeep a secret, so everyone knows about this. That is the source of muchconfusion. It's better to pretend that the internal format is some unknownencoding, 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 itmakes no sense to deal with characters in a byte string. Do the properconversions (by decoding/encoding), and things will work out well: you getcharacter counts for decoded data, and byte counts for encoded data.

use bytes is usually a failed attempt to do something useful. Just forgetabout it.

What about the `use encoding` pragma?

Don't use it. Unfortunately, it assumes that the programmer's environment andthat of the user will use the same encoding. It will use the same encoding forthe source code and for STDIN and STDOUT. When a program is copied to anothermachine, 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 encodedfile and use utf8.

If you need to set the encoding for STDIN, STDOUT, and STDERR, for examplebased 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 theencoding or decoding step, and manipulate the UTF8 flag directly.

Instead of :encoding(UTF-8), you can simply use :utf8, which skips theencoding step if the data was already represented as UTF8 internally. This iswidely accepted as good behavior when you're writing, but it can be dangerouswhen reading, because it causes internal inconsistency when you have invalidbyte sequences. Using :utf8 for input can sometimes result in securitybreaches, 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, forthe 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 inwhat 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 thingsthat are too liberal, you may have to use utf8. The full explanation is inEncode.

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

For example, utf8 can be used for code points that don't exist in Unicode, like9999999, but if you encode that to UTF-8, you get a substitution character (bydefault; see Handling Malformed Data in Encode for more ways of dealing withthis.)

Okay, if you insist: the "internal format" is utf8, not UTF-8. (When it's notsome 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 internalformat being any specific encoding. But since you asked: by default, theinternal format is either ISO-8859-1 (latin-1), or utf8, depending on thehistory of the string. On EBCDIC platforms, this may be different even.

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

AUTHOR

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

Perl Unicode FAQ

Daftar Isi

NAME

Q and A

perlunitut isn't really a Unicode tutorial, is it?