Localize $! in splainthis() too. (see bug #41717)

[p5sagit/p5-mst-13.2.git] / pod / perlunitut.pod

=head1 NAME

perlunitut - Perl Unicode Tutorial

=head1 DESCRIPTION

The days of just flinging strings around are over. It's well established that
modern programs need to be capable of communicating funny accented letters, and
things like euro symbols. This means that programmers need new habits. It's
easy to program Unicode capable software, but it does require discipline to do
it right.

There's a lot to know about character sets, and text encodings. It's probably
best to spend a full day learning all this, but the basics can be learned in
minutes. 

These are not the very basics, though. It is assumed that you already
know the difference between bytes and characters, and realise (and accept!)
that there are many different character sets and encodings, and that your
program has to be explicit about them. Recommended reading is "The Absolute
Minimum Every Software Developer Absolutely, Positively Must Know About Unicode
and Character Sets (No Excuses!)" by Joel Spolsky, at
L<http://joelonsoftware.com/articles/Unicode.html>.

This tutorial speaks in rather absolute terms, and provides only a limited view
of the wealth of character string related features that Perl has to offer. For
most projects, this information will probably suffice.

=head2 Definitions

It's important to set a few things straight first. This is the most important
part of this tutorial. This view may conflict with other information that you
may have found on the web, but that's mostly because many sources are wrong.

You may have to re-read this entire section a few times...

=head3 Unicode

B<Unicode> is a character set with room for lots of characters. The ordinal
value of a character is called a B<code point>. 

There are many, many code points, but computers work with bytes, and a byte can
have only 256 values. Unicode has many more characters, so you need a method
to make these accessible.

Unicode is encoded using several competing encodings, of which UTF-8 is the
most used. In a Unicode encoding, multiple subsequent bytes can be used to
store a single code point, or simply: character.

=head3 UTF-8

B<UTF-8> is a Unicode encoding. Many people think that Unicode and UTF-8 are
the same thing, but they're not. There are more Unicode encodings, but much of
the world has standardized on UTF-8. 

UTF-8 treats the first 128 codepoints, 0..127, the same as ASCII. They take
only one byte per character. All other characters are encoded as two or more
(up to six) bytes using a complex scheme. Fortunately, Perl handles this for
us, so we don't have to worry about this.

=head3 Text strings (character strings)

B<Text strings>, or B<character strings> are made of characters. Bytes are
irrelevant here, and so are encodings. Each character is just that: the
character.

On a text string, you would do things like:

    $text =~ s/foo/bar/;
    if ($string =~ /^\d+$/) { ... }
    $text = ucfirst $text;
    my $character_count = length $text;

The value of a character (C<ord>, C<chr>) is the corresponding Unicode code
point.

=head3 Binary strings (byte strings)

B<Binary strings>, or B<byte strings> are made of bytes. Here, you don't have
characters, just bytes. All communication with the outside world (anything
outside of your current Perl process) is done in binary.

On a binary string, you would do things like:

    my (@length_content) = unpack "(V/a)*", $binary;
    $binary =~ s/\x00\x0F/\xFF\xF0/;  # for the brave :)
    print {$fh} $binary;
    my $byte_count = length $binary;

=head3 Encoding

B<Encoding> (as a verb) is the conversion from I<text> to I<binary>. To encode,
you have to supply the target encoding, for example C<iso-8859-1> or C<UTF-8>.
Some encodings, like the C<iso-8859> ("latin") range, do not support the full
Unicode standard; characters that can't be represented are lost in the
conversion.

=head3 Decoding

B<Decoding> is the conversion from I<binary> to I<text>. To decode, you have to
know what encoding was used during the encoding phase. And most of all, it must
be something decodable. It doesn't make much sense to decode a PNG image into a
text string.

=head3 Internal format

Perl has an B<internal format>, an encoding that it uses to encode text strings
so it can store them in memory. All text strings are in this internal format.
In fact, text strings are never in any other format!

You shouldn't worry about what this format is, because conversion is
automatically done when you decode or encode.

=head2 Your new toolkit

Add to your standard heading the following line:

    use Encode qw(encode decode);

Or, if you're lazy, just:

    use Encode;

=head2 I/O flow (the actual 5 minute tutorial)

The typical input/output flow of a program is:

    1. Receive and decode
    2. Process
    3. Encode and output

If your input is binary, and is supposed to remain binary, you shouldn't decode
it to a text string, of course. But in all other cases, you should decode it.

Decoding can't happen reliably if you don't know how the data was encoded. If
you get to choose, it's a good idea to standardize on UTF-8.

    my $foo   = decode('UTF-8', get 'http://example.com/');
    my $bar   = decode('ISO-8859-1', readline STDIN);
    my $xyzzy = decode('Windows-1251', $cgi->param('foo'));

Processing happens as you knew before. The only difference is that you're now
using characters instead of bytes. That's very useful if you use things like
C<substr>, or C<length>.

It's important to realize that there are no bytes in a text string. Of course,
Perl has its internal encoding to store the string in memory, but ignore that.
If you have to do anything with the number of bytes, it's probably best to move
that part to step 3, just after you've encoded the string. Then you know
exactly how many bytes it will be in the destination string.

The syntax for encoding text strings to binary strings is as simple as decoding:

    $body = encode('UTF-8', $body);

If you needed to know the length of the string in bytes, now's the perfect time
for that. Because C<$body> is now a byte string, C<length> will report the
number of bytes, instead of the number of characters. The number of
characters is no longer known, because characters only exist in text strings.

    my $byte_count = length $body;

And if the protocol you're using supports a way of letting the recipient know
which character encoding you used, please help the receiving end by using that
feature! For example, E-mail and HTTP support MIME headers, so you can use the
C<Content-Type> header. They can also have C<Content-Length> to indicate the
number of I<bytes>, which is always a good idea to supply if the number is
known.

    "Content-Type: text/plain; charset=UTF-8",
    "Content-Length: $byte_count"

=head2 Q and A

=head3 This isn't really a Unicode tutorial, is it?

No, Perl has an abstracted interface for all supported character encodings, so
this is actually a generic C<Encode> tutorial. But many people think that
Unicode is special and magical, and I didn't want to disappoint them, so I
decided to call this document a Unicode tutorial.

=head3 What about binary data, like images?

Well, apart from a bare C<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?".

=head3 What about the UTF-8 flag?

Please, unless you're hacking the internals, or debugging weirdness, don't
think about the UTF-8 flag at all. That means that you very probably shouldn't
use C<is_utf8>, C<_utf8_on> or C<_utf8_off> at all.

Perl's internal format 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.

=head3 When should I decode or encode?

Whenever you're communicating 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.

=head3 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 (C<&amp;gt;>), or double URI encoding
(C<%253E>).

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

=head3 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.

=head3 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 C<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 C<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 typically limited to the UTF-8 encoding, because they cheat.

=head3 Cheat?! Tell me, how can I cheat?

Well, because Perl's internal format is UTF-8, you can just skip the encoding
or decoding step, and manipulate the UTF-8 flag directly.

Instead of C<:encoding(UTF-8)>, you can simply use C<:utf8>. This is widely
accepted as good behavior.

Instead of C<decode> and C<encode>, you could use C<_utf8_on> and C<_utf8_off>.
But this is, contrary to C<:utf8>, considered bad style.

There are some shortcuts for oneliners; see C<-C> in L<perlrun>.

=head3 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.

=head3 Can I use Unicode in my Perl sources?

Yes, you can! If your sources are UTF-8 encoded, you can indicate that with the
C<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 C<\w>),
and even in custom delimiters.

=head3 Data::Dumper doesn't restore the UTF-8 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 UTF-8 flag when the data is read
again with C<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 C<eval> dumped data as always.

=head3 How can I determine if a string is a text string or a binary string?

You can't. Some use the UTF-8 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.

=head3 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>;

=head3 What about the C<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.

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

=head3 What are C<decode_utf8> and C<encode_utf8>?

These are alternate syntaxes for C<decode('utf8', ...)> and C<encode('utf8',
...)>.

=head3 What's the difference between C<UTF-8> and C<utf8>?

C<UTF-8> is the official standard. C<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 C<UTF-8>. If you have to communicate with things
that are too liberal, you may have to use C<utf8>. The full explanation is in
L<Encode>.

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

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

=head3 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.

Perl knows how it stored the string internally, and will use that knowledge
when you C<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.

=head3 What character encodings does Perl support?

To find out which character encodings your Perl supports, run:

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

=head3 Which version of perl should I use?

Well, if you can, upgrade to the most recent, but certainly C<5.8.1> or newer.
This tutorial is based on the status quo as of C<5.8.7>.

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.

=head1 SUMMARY

Decode everything you receive, encode everything you send out. (If it's text
data.)

=head1 ACKNOWLEDGEMENTS

Thanks to Johan Vromans from Squirrel Consultancy. His UTF-8 rants during the
Amsterdam Perl Mongers meetings got me interested and determined to find out
how to use character encodings in Perl in ways that don't break easily.

Thanks to Gerard Goossen from TTY. His presentation "UTF-8 in the wild" (Dutch
Perl Workshop 2006) inspired me to publish my thoughts and write this tutorial.

Thanks to the people who asked about this kind of stuff in several Perl IRC
channels, and have constantly reminded me that a simpler explanation was
needed.

Thanks to the people who reviewed this document for me, before it went public.
They are: Benjamin Smith, Jan-Pieter Cornet, Johan Vromans, Lukas Mai, Nathan
Gray.

=head1 AUTHOR

Juerd Waalboer <juerd@cpan.org>

=head1 SEE ALSO

L<perlunicode>, L<perluniintro>, L<Encode>