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1 | =head1 NAME |
2 | ||
3 | perlunitut - Perl Unicode Tutorial | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
7 | The days of just flinging strings around are over. It's well established that | |
8 | modern programs need to be capable of communicating funny accented letters, and | |
9 | things like euro symbols. This means that programmers need new habits. It's | |
10 | easy to program Unicode capable software, but it does require discipline to do | |
11 | it right. | |
12 | ||
13 | There's a lot to know about character sets, and text encodings. It's probably | |
14 | best to spend a full day learning all this, but the basics can be learned in | |
15 | minutes. | |
16 | ||
17 | These are not the very basics, though. It is assumed that you already | |
18 | know the difference between bytes and characters, and realise (and accept!) | |
19 | that there are many different character sets and encodings, and that your | |
20 | program has to be explicit about them. Recommended reading is "The Absolute | |
21 | Minimum Every Software Developer Absolutely, Positively Must Know About Unicode | |
22 | and Character Sets (No Excuses!)" by Joel Spolsky, at | |
23 | L<http://joelonsoftware.com/articles/Unicode.html>. | |
24 | ||
25 | This tutorial speaks in rather absolute terms, and provides only a limited view | |
26 | of the wealth of character string related features that Perl has to offer. For | |
27 | most projects, this information will probably suffice. | |
28 | ||
29 | =head2 Definitions | |
30 | ||
31 | It's important to set a few things straight first. This is the most important | |
32 | part of this tutorial. This view may conflict with other information that you | |
33 | may have found on the web, but that's mostly because many sources are wrong. | |
34 | ||
35 | You may have to re-read this entire section a few times... | |
36 | ||
37 | =head3 Unicode | |
38 | ||
39 | B<Unicode> is a character set with room for lots of characters. The ordinal | |
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40 | value of a character is called a B<code point>. (But in practice, the |
41 | distinction between code point and character is blurred, so the terms often | |
42 | are used interchangeably.) | |
aadaa455 | 43 | |
e1b711da | 44 | There are many, many code points, but computers work with bytes, and a byte has |
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45 | room for only 256 values. Unicode has many more characters than that, |
46 | so you need a method to make these accessible. | |
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47 | |
48 | Unicode is encoded using several competing encodings, of which UTF-8 is the | |
49 | most used. In a Unicode encoding, multiple subsequent bytes can be used to | |
50 | store a single code point, or simply: character. | |
51 | ||
52 | =head3 UTF-8 | |
53 | ||
54 | B<UTF-8> is a Unicode encoding. Many people think that Unicode and UTF-8 are | |
55 | the same thing, but they're not. There are more Unicode encodings, but much of | |
56 | the world has standardized on UTF-8. | |
57 | ||
58 | UTF-8 treats the first 128 codepoints, 0..127, the same as ASCII. They take | |
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59 | only one byte per character. All other characters are encoded as two to |
60 | four bytes using a complex scheme. Fortunately, Perl handles this for | |
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61 | us, so we don't have to worry about this. |
62 | ||
63 | =head3 Text strings (character strings) | |
64 | ||
65 | B<Text strings>, or B<character strings> are made of characters. Bytes are | |
66 | irrelevant here, and so are encodings. Each character is just that: the | |
67 | character. | |
68 | ||
69 | On a text string, you would do things like: | |
70 | ||
71 | $text =~ s/foo/bar/; | |
72 | if ($string =~ /^\d+$/) { ... } | |
73 | $text = ucfirst $text; | |
74 | my $character_count = length $text; | |
75 | ||
76 | The value of a character (C<ord>, C<chr>) is the corresponding Unicode code | |
77 | point. | |
78 | ||
79 | =head3 Binary strings (byte strings) | |
80 | ||
81 | B<Binary strings>, or B<byte strings> are made of bytes. Here, you don't have | |
82 | characters, just bytes. All communication with the outside world (anything | |
83 | outside of your current Perl process) is done in binary. | |
84 | ||
85 | On a binary string, you would do things like: | |
86 | ||
87 | my (@length_content) = unpack "(V/a)*", $binary; | |
88 | $binary =~ s/\x00\x0F/\xFF\xF0/; # for the brave :) | |
89 | print {$fh} $binary; | |
90 | my $byte_count = length $binary; | |
91 | ||
92 | =head3 Encoding | |
93 | ||
94 | B<Encoding> (as a verb) is the conversion from I<text> to I<binary>. To encode, | |
95 | you have to supply the target encoding, for example C<iso-8859-1> or C<UTF-8>. | |
96 | Some encodings, like the C<iso-8859> ("latin") range, do not support the full | |
97 | Unicode standard; characters that can't be represented are lost in the | |
98 | conversion. | |
99 | ||
100 | =head3 Decoding | |
101 | ||
102 | B<Decoding> is the conversion from I<binary> to I<text>. To decode, you have to | |
103 | know what encoding was used during the encoding phase. And most of all, it must | |
104 | be something decodable. It doesn't make much sense to decode a PNG image into a | |
105 | text string. | |
106 | ||
107 | =head3 Internal format | |
108 | ||
109 | Perl has an B<internal format>, an encoding that it uses to encode text strings | |
110 | so it can store them in memory. All text strings are in this internal format. | |
111 | In fact, text strings are never in any other format! | |
112 | ||
113 | You shouldn't worry about what this format is, because conversion is | |
114 | automatically done when you decode or encode. | |
115 | ||
116 | =head2 Your new toolkit | |
117 | ||
118 | Add to your standard heading the following line: | |
119 | ||
120 | use Encode qw(encode decode); | |
121 | ||
122 | Or, if you're lazy, just: | |
123 | ||
124 | use Encode; | |
125 | ||
126 | =head2 I/O flow (the actual 5 minute tutorial) | |
127 | ||
128 | The typical input/output flow of a program is: | |
129 | ||
130 | 1. Receive and decode | |
131 | 2. Process | |
132 | 3. Encode and output | |
133 | ||
134 | If your input is binary, and is supposed to remain binary, you shouldn't decode | |
135 | it to a text string, of course. But in all other cases, you should decode it. | |
136 | ||
137 | Decoding can't happen reliably if you don't know how the data was encoded. If | |
138 | you get to choose, it's a good idea to standardize on UTF-8. | |
139 | ||
140 | my $foo = decode('UTF-8', get 'http://example.com/'); | |
141 | my $bar = decode('ISO-8859-1', readline STDIN); | |
142 | my $xyzzy = decode('Windows-1251', $cgi->param('foo')); | |
143 | ||
144 | Processing happens as you knew before. The only difference is that you're now | |
145 | using characters instead of bytes. That's very useful if you use things like | |
146 | C<substr>, or C<length>. | |
147 | ||
148 | It's important to realize that there are no bytes in a text string. Of course, | |
149 | Perl has its internal encoding to store the string in memory, but ignore that. | |
150 | If you have to do anything with the number of bytes, it's probably best to move | |
151 | that part to step 3, just after you've encoded the string. Then you know | |
152 | exactly how many bytes it will be in the destination string. | |
153 | ||
154 | The syntax for encoding text strings to binary strings is as simple as decoding: | |
155 | ||
156 | $body = encode('UTF-8', $body); | |
157 | ||
158 | If you needed to know the length of the string in bytes, now's the perfect time | |
159 | for that. Because C<$body> is now a byte string, C<length> will report the | |
160 | number of bytes, instead of the number of characters. The number of | |
161 | characters is no longer known, because characters only exist in text strings. | |
162 | ||
163 | my $byte_count = length $body; | |
164 | ||
165 | And if the protocol you're using supports a way of letting the recipient know | |
166 | which character encoding you used, please help the receiving end by using that | |
167 | feature! For example, E-mail and HTTP support MIME headers, so you can use the | |
168 | C<Content-Type> header. They can also have C<Content-Length> to indicate the | |
169 | number of I<bytes>, which is always a good idea to supply if the number is | |
170 | known. | |
171 | ||
172 | "Content-Type: text/plain; charset=UTF-8", | |
173 | "Content-Length: $byte_count" | |
174 | ||
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175 | =head1 SUMMARY |
176 | ||
177 | Decode everything you receive, encode everything you send out. (If it's text | |
178 | data.) | |
179 | ||
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180 | =head1 Q and A (or FAQ) |
181 | ||
182 | After reading this document, you ought to read L<perlunifaq> too. | |
183 | ||
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184 | =head1 ACKNOWLEDGEMENTS |
185 | ||
186 | Thanks to Johan Vromans from Squirrel Consultancy. His UTF-8 rants during the | |
187 | Amsterdam Perl Mongers meetings got me interested and determined to find out | |
188 | how to use character encodings in Perl in ways that don't break easily. | |
189 | ||
190 | Thanks to Gerard Goossen from TTY. His presentation "UTF-8 in the wild" (Dutch | |
191 | Perl Workshop 2006) inspired me to publish my thoughts and write this tutorial. | |
192 | ||
193 | Thanks to the people who asked about this kind of stuff in several Perl IRC | |
194 | channels, and have constantly reminded me that a simpler explanation was | |
195 | needed. | |
196 | ||
197 | Thanks to the people who reviewed this document for me, before it went public. | |
198 | They are: Benjamin Smith, Jan-Pieter Cornet, Johan Vromans, Lukas Mai, Nathan | |
199 | Gray. | |
200 | ||
201 | =head1 AUTHOR | |
202 | ||
740d4bb2 | 203 | Juerd Waalboer <#####@juerd.nl> |
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204 | |
205 | =head1 SEE ALSO | |
206 | ||
2575c402 | 207 | L<perlunifaq>, L<perlunicode>, L<perluniintro>, L<Encode> |
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