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1=head1 NAME
2
3perlunitut - Perl Unicode Tutorial
4
5=head1 DESCRIPTION
6
7The days of just flinging strings around are over. It's well established that
8modern programs need to be capable of communicating funny accented letters, and
9things like euro symbols. This means that programmers need new habits. It's
10easy to program Unicode capable software, but it does require discipline to do
11it right.
12
13There's a lot to know about character sets, and text encodings. It's probably
14best to spend a full day learning all this, but the basics can be learned in
15minutes.
16
17These are not the very basics, though. It is assumed that you already
18know the difference between bytes and characters, and realise (and accept!)
19that there are many different character sets and encodings, and that your
20program has to be explicit about them. Recommended reading is "The Absolute
21Minimum Every Software Developer Absolutely, Positively Must Know About Unicode
22and Character Sets (No Excuses!)" by Joel Spolsky, at
23L<http://joelonsoftware.com/articles/Unicode.html>.
24
25This tutorial speaks in rather absolute terms, and provides only a limited view
26of the wealth of character string related features that Perl has to offer. For
27most projects, this information will probably suffice.
28
29=head2 Definitions
30
31It's important to set a few things straight first. This is the most important
32part of this tutorial. This view may conflict with other information that you
33may have found on the web, but that's mostly because many sources are wrong.
34
35You may have to re-read this entire section a few times...
36
37=head3 Unicode
38
39B<Unicode> is a character set with room for lots of characters. The ordinal
40value of a character is called a B<code point>.
41
42There are many, many code points, but computers work with bytes, and a byte can
43have only 256 values. Unicode has many more characters, so you need a method
44to make these accessible.
45
46Unicode is encoded using several competing encodings, of which UTF-8 is the
47most used. In a Unicode encoding, multiple subsequent bytes can be used to
48store a single code point, or simply: character.
49
50=head3 UTF-8
51
52B<UTF-8> is a Unicode encoding. Many people think that Unicode and UTF-8 are
53the same thing, but they're not. There are more Unicode encodings, but much of
54the world has standardized on UTF-8.
55
56UTF-8 treats the first 128 codepoints, 0..127, the same as ASCII. They take
57only one byte per character. All other characters are encoded as two or more
58(up to six) bytes using a complex scheme. Fortunately, Perl handles this for
59us, so we don't have to worry about this.
60
61=head3 Text strings (character strings)
62
63B<Text strings>, or B<character strings> are made of characters. Bytes are
64irrelevant here, and so are encodings. Each character is just that: the
65character.
66
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67Text strings are also called B<Unicode strings>, because in Perl, every text
68string is a Unicode string.
69
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70On a text string, you would do things like:
71
72 $text =~ s/foo/bar/;
73 if ($string =~ /^\d+$/) { ... }
74 $text = ucfirst $text;
75 my $character_count = length $text;
76
77The value of a character (C<ord>, C<chr>) is the corresponding Unicode code
78point.
79
80=head3 Binary strings (byte strings)
81
82B<Binary strings>, or B<byte strings> are made of bytes. Here, you don't have
83characters, just bytes. All communication with the outside world (anything
84outside of your current Perl process) is done in binary.
85
86On a binary string, you would do things like:
87
88 my (@length_content) = unpack "(V/a)*", $binary;
89 $binary =~ s/\x00\x0F/\xFF\xF0/; # for the brave :)
90 print {$fh} $binary;
91 my $byte_count = length $binary;
92
93=head3 Encoding
94
95B<Encoding> (as a verb) is the conversion from I<text> to I<binary>. To encode,
96you have to supply the target encoding, for example C<iso-8859-1> or C<UTF-8>.
97Some encodings, like the C<iso-8859> ("latin") range, do not support the full
98Unicode standard; characters that can't be represented are lost in the
99conversion.
100
101=head3 Decoding
102
103B<Decoding> is the conversion from I<binary> to I<text>. To decode, you have to
104know what encoding was used during the encoding phase. And most of all, it must
105be something decodable. It doesn't make much sense to decode a PNG image into a
106text string.
107
108=head3 Internal format
109
110Perl has an B<internal format>, an encoding that it uses to encode text strings
111so it can store them in memory. All text strings are in this internal format.
112In fact, text strings are never in any other format!
113
114You shouldn't worry about what this format is, because conversion is
115automatically done when you decode or encode.
116
117=head2 Your new toolkit
118
119Add to your standard heading the following line:
120
121 use Encode qw(encode decode);
122
123Or, if you're lazy, just:
124
125 use Encode;
126
127=head2 I/O flow (the actual 5 minute tutorial)
128
129The typical input/output flow of a program is:
130
131 1. Receive and decode
132 2. Process
133 3. Encode and output
134
135If your input is binary, and is supposed to remain binary, you shouldn't decode
136it to a text string, of course. But in all other cases, you should decode it.
137
138Decoding can't happen reliably if you don't know how the data was encoded. If
139you get to choose, it's a good idea to standardize on UTF-8.
140
141 my $foo = decode('UTF-8', get 'http://example.com/');
142 my $bar = decode('ISO-8859-1', readline STDIN);
143 my $xyzzy = decode('Windows-1251', $cgi->param('foo'));
144
145Processing happens as you knew before. The only difference is that you're now
146using characters instead of bytes. That's very useful if you use things like
147C<substr>, or C<length>.
148
149It's important to realize that there are no bytes in a text string. Of course,
150Perl has its internal encoding to store the string in memory, but ignore that.
151If you have to do anything with the number of bytes, it's probably best to move
152that part to step 3, just after you've encoded the string. Then you know
153exactly how many bytes it will be in the destination string.
154
155The syntax for encoding text strings to binary strings is as simple as decoding:
156
157 $body = encode('UTF-8', $body);
158
159If you needed to know the length of the string in bytes, now's the perfect time
160for that. Because C<$body> is now a byte string, C<length> will report the
161number of bytes, instead of the number of characters. The number of
162characters is no longer known, because characters only exist in text strings.
163
164 my $byte_count = length $body;
165
166And if the protocol you're using supports a way of letting the recipient know
167which character encoding you used, please help the receiving end by using that
168feature! For example, E-mail and HTTP support MIME headers, so you can use the
169C<Content-Type> header. They can also have C<Content-Length> to indicate the
170number of I<bytes>, which is always a good idea to supply if the number is
171known.
172
173 "Content-Type: text/plain; charset=UTF-8",
174 "Content-Length: $byte_count"
175
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176=head1 SUMMARY
177
178Decode everything you receive, encode everything you send out. (If it's text
179data.)
180
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181=head1 Q and A (or FAQ)
182
183After reading this document, you ought to read L<perlunifaq> too.
184
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185=head1 ACKNOWLEDGEMENTS
186
187Thanks to Johan Vromans from Squirrel Consultancy. His UTF-8 rants during the
188Amsterdam Perl Mongers meetings got me interested and determined to find out
189how to use character encodings in Perl in ways that don't break easily.
190
191Thanks to Gerard Goossen from TTY. His presentation "UTF-8 in the wild" (Dutch
192Perl Workshop 2006) inspired me to publish my thoughts and write this tutorial.
193
194Thanks to the people who asked about this kind of stuff in several Perl IRC
195channels, and have constantly reminded me that a simpler explanation was
196needed.
197
198Thanks to the people who reviewed this document for me, before it went public.
199They are: Benjamin Smith, Jan-Pieter Cornet, Johan Vromans, Lukas Mai, Nathan
200Gray.
201
202=head1 AUTHOR
203
740d4bb2 204Juerd Waalboer <#####@juerd.nl>
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205
206=head1 SEE ALSO
207
2575c402 208L<perlunifaq>, L<perlunicode>, L<perluniintro>, L<Encode>
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