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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 |
40 | value of a character is called a B<code point>. |
41 | |
42 | There are many, many code points, but computers work with bytes, and a byte can |
43 | have only 256 values. Unicode has many more characters, so you need a method |
44 | to make these accessible. |
45 | |
46 | Unicode is encoded using several competing encodings, of which UTF-8 is the |
47 | most used. In a Unicode encoding, multiple subsequent bytes can be used to |
48 | store a single code point, or simply: character. |
49 | |
50 | =head3 UTF-8 |
51 | |
52 | B<UTF-8> is a Unicode encoding. Many people think that Unicode and UTF-8 are |
53 | the same thing, but they're not. There are more Unicode encodings, but much of |
54 | the world has standardized on UTF-8. |
55 | |
56 | UTF-8 treats the first 128 codepoints, 0..127, the same as ASCII. They take |
57 | only 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 |
59 | us, so we don't have to worry about this. |
60 | |
61 | =head3 Text strings (character strings) |
62 | |
63 | B<Text strings>, or B<character strings> are made of characters. Bytes are |
64 | irrelevant here, and so are encodings. Each character is just that: the |
65 | character. |
66 | |
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67 | Text strings are also called B<Unicode strings>, because in Perl, every text |
68 | string is a Unicode string. |
69 | |
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70 | On 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 | |
77 | The value of a character (C<ord>, C<chr>) is the corresponding Unicode code |
78 | point. |
79 | |
80 | =head3 Binary strings (byte strings) |
81 | |
82 | B<Binary strings>, or B<byte strings> are made of bytes. Here, you don't have |
83 | characters, just bytes. All communication with the outside world (anything |
84 | outside of your current Perl process) is done in binary. |
85 | |
86 | On 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 | |
95 | B<Encoding> (as a verb) is the conversion from I<text> to I<binary>. To encode, |
96 | you have to supply the target encoding, for example C<iso-8859-1> or C<UTF-8>. |
97 | Some encodings, like the C<iso-8859> ("latin") range, do not support the full |
98 | Unicode standard; characters that can't be represented are lost in the |
99 | conversion. |
100 | |
101 | =head3 Decoding |
102 | |
103 | B<Decoding> is the conversion from I<binary> to I<text>. To decode, you have to |
104 | know what encoding was used during the encoding phase. And most of all, it must |
105 | be something decodable. It doesn't make much sense to decode a PNG image into a |
106 | text string. |
107 | |
108 | =head3 Internal format |
109 | |
110 | Perl has an B<internal format>, an encoding that it uses to encode text strings |
111 | so it can store them in memory. All text strings are in this internal format. |
112 | In fact, text strings are never in any other format! |
113 | |
114 | You shouldn't worry about what this format is, because conversion is |
115 | automatically done when you decode or encode. |
116 | |
117 | =head2 Your new toolkit |
118 | |
119 | Add to your standard heading the following line: |
120 | |
121 | use Encode qw(encode decode); |
122 | |
123 | Or, if you're lazy, just: |
124 | |
125 | use Encode; |
126 | |
127 | =head2 I/O flow (the actual 5 minute tutorial) |
128 | |
129 | The typical input/output flow of a program is: |
130 | |
131 | 1. Receive and decode |
132 | 2. Process |
133 | 3. Encode and output |
134 | |
135 | If your input is binary, and is supposed to remain binary, you shouldn't decode |
136 | it to a text string, of course. But in all other cases, you should decode it. |
137 | |
138 | Decoding can't happen reliably if you don't know how the data was encoded. If |
139 | you 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 | |
145 | Processing happens as you knew before. The only difference is that you're now |
146 | using characters instead of bytes. That's very useful if you use things like |
147 | C<substr>, or C<length>. |
148 | |
149 | It's important to realize that there are no bytes in a text string. Of course, |
150 | Perl has its internal encoding to store the string in memory, but ignore that. |
151 | If you have to do anything with the number of bytes, it's probably best to move |
152 | that part to step 3, just after you've encoded the string. Then you know |
153 | exactly how many bytes it will be in the destination string. |
154 | |
155 | The syntax for encoding text strings to binary strings is as simple as decoding: |
156 | |
157 | $body = encode('UTF-8', $body); |
158 | |
159 | If you needed to know the length of the string in bytes, now's the perfect time |
160 | for that. Because C<$body> is now a byte string, C<length> will report the |
161 | number of bytes, instead of the number of characters. The number of |
162 | characters is no longer known, because characters only exist in text strings. |
163 | |
164 | my $byte_count = length $body; |
165 | |
166 | And if the protocol you're using supports a way of letting the recipient know |
167 | which character encoding you used, please help the receiving end by using that |
168 | feature! For example, E-mail and HTTP support MIME headers, so you can use the |
169 | C<Content-Type> header. They can also have C<Content-Length> to indicate the |
170 | number of I<bytes>, which is always a good idea to supply if the number is |
171 | known. |
172 | |
173 | "Content-Type: text/plain; charset=UTF-8", |
174 | "Content-Length: $byte_count" |
175 | |
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176 | =head1 SUMMARY |
177 | |
178 | Decode everything you receive, encode everything you send out. (If it's text |
179 | data.) |
180 | |
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181 | =head1 Q and A (or FAQ) |
182 | |
183 | After reading this document, you ought to read L<perlunifaq> too. |
184 | |
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185 | =head1 ACKNOWLEDGEMENTS |
186 | |
187 | Thanks to Johan Vromans from Squirrel Consultancy. His UTF-8 rants during the |
188 | Amsterdam Perl Mongers meetings got me interested and determined to find out |
189 | how to use character encodings in Perl in ways that don't break easily. |
190 | |
191 | Thanks to Gerard Goossen from TTY. His presentation "UTF-8 in the wild" (Dutch |
192 | Perl Workshop 2006) inspired me to publish my thoughts and write this tutorial. |
193 | |
194 | Thanks to the people who asked about this kind of stuff in several Perl IRC |
195 | channels, and have constantly reminded me that a simpler explanation was |
196 | needed. |
197 | |
198 | Thanks to the people who reviewed this document for me, before it went public. |
199 | They are: Benjamin Smith, Jan-Pieter Cornet, Johan Vromans, Lukas Mai, Nathan |
200 | Gray. |
201 | |
202 | =head1 AUTHOR |
203 | |
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204 | Juerd Waalboer <#####@juerd.nl> |
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205 | |
206 | =head1 SEE ALSO |
207 | |
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208 | L<perlunifaq>, L<perlunicode>, L<perluniintro>, L<Encode> |
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209 | |