1PERLUNITUT(1) Perl Programmers Reference Guide PERLUNITUT(1)
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6 perlunitut - Perl Unicode Tutorial
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9 The days of just flinging strings around are over. It's well
10 established that modern programs need to be capable of communicating
11 funny accented letters, and things like euro symbols. This means that
12 programmers need new habits. It's easy to program Unicode capable
13 software, but it does require discipline to do it right.
14 There's a lot to know about character sets, and text encodings. It's
16 probably best to spend a full day learning all this, but the basics can
17 be learned in minutes.
18 These are not the very basics, though. It is assumed that you already
20 know the difference between bytes and characters, and realise (and
21 accept!) that there are many different character sets and encodings,
22 and that your program has to be explicit about them. Recommended
23 reading is "The Absolute Minimum Every Software Developer Absolutely,
24 Positively Must Know About Unicode and Character Sets (No Excuses!)" by
25 Joel Spolsky, at <http://joelonsoftware.com/articles/Unicode.html>.
26 This tutorial speaks in rather absolute terms, and provides only a
28 limited view of the wealth of character string related features that
29 Perl has to offer. For most projects, this information will probably
30 suffice.
31 Definitions
33 It's important to set a few things straight first. This is the most
34 important part of this tutorial. This view may conflict with other
35 information that you may have found on the web, but that's mostly
36 because many sources are wrong.
37 You may have to re-read this entire section a few times...
39 Unicode
41 Unicode is a character set with room for lots of characters. The
43 ordinal value of a character is called a code point. (But in
44 practice, the distinction between code point and character is blurred,
45 so the terms often are used interchangeably.)
46 There are many, many code points, but computers work with bytes, and a
48 byte has room for only 256 values. Unicode has many more characters
49 than that, so you need a method to make these accessible.
50 Unicode is encoded using several competing encodings, of which UTF-8 is
52 the most used. In a Unicode encoding, multiple subsequent bytes can be
53 used to store a single code point, or simply: character.
54 UTF-8
56 UTF-8 is a Unicode encoding. Many people think that Unicode and UTF-8
58 are the same thing, but they're not. There are more Unicode encodings,
59 but much of the world has standardized on UTF-8.
60 UTF-8 treats the first 128 codepoints, 0..127, the same as ASCII. They
62 take only one byte per character. All other characters are encoded as
63 two or more (up to six) bytes using a complex scheme. Fortunately, Perl
64 handles this for us, so we don't have to worry about this.
65 Text strings (character strings)
67 Text strings, or character strings are made of characters. Bytes are
69 irrelevant here, and so are encodings. Each character is just that: the
70 character.
71 On a text string, you would do things like:
73 $text =~ s/foo/bar/;
75 if ($string =~ /^\d+$/) { ... }
76 $text = ucfirst $text;
77 my $character_count = length $text;
78 The value of a character ("ord", "chr") is the corresponding Unicode
80 code point.
81 Binary strings (byte strings)
83 Binary strings, or byte strings are made of bytes. Here, you don't have
85 characters, just bytes. All communication with the outside world
86 (anything outside of your current Perl process) is done in binary.
87 On a binary string, you would do things like:
89 my (@length_content) = unpack "(V/a)*", $binary;
91 $binary =~ s/\x00\x0F/\xFF\xF0/; # for the brave :)
92 print {$fh} $binary;
93 my $byte_count = length $binary;
94 Encoding
96 Encoding (as a verb) is the conversion from text to binary. To encode,
98 you have to supply the target encoding, for example "iso-8859-1" or
99 "UTF-8". Some encodings, like the "iso-8859" ("latin") range, do not
100 support the full Unicode standard; characters that can't be represented
101 are lost in the conversion.
102 Decoding
104 Decoding is the conversion from binary to text. To decode, you have to
106 know what encoding was used during the encoding phase. And most of all,
107 it must be something decodable. It doesn't make much sense to decode a
108 PNG image into a text string.
109 Internal format
111 Perl has an internal format, an encoding that it uses to encode text
113 strings so it can store them in memory. All text strings are in this
114 internal format. In fact, text strings are never in any other format!
115 You shouldn't worry about what this format is, because conversion is
117 automatically done when you decode or encode.
118 Your new toolkit
120 Add to your standard heading the following line:
121 use Encode qw(encode decode);
123 Or, if you're lazy, just:
125 use Encode;
127 I/O flow (the actual 5 minute tutorial)
129 The typical input/output flow of a program is:
130 1. Receive and decode
132 2. Process
133 3. Encode and output
134 If your input is binary, and is supposed to remain binary, you
136 shouldn't decode it to a text string, of course. But in all other
137 cases, you should decode it.
138 Decoding can't happen reliably if you don't know how the data was
140 encoded. If you get to choose, it's a good idea to standardize on
141 UTF-8.
142 my $foo = decode('UTF-8', get 'http://example.com/');
144 my $bar = decode('ISO-8859-1', readline STDIN);
145 my $xyzzy = decode('Windows-1251', $cgi->param('foo'));
146 Processing happens as you knew before. The only difference is that
148 you're now using characters instead of bytes. That's very useful if you
149 use things like "substr", or "length".
150 It's important to realize that there are no bytes in a text string. Of
152 course, Perl has its internal encoding to store the string in memory,
153 but ignore that. If you have to do anything with the number of bytes,
154 it's probably best to move that part to step 3, just after you've
155 encoded the string. Then you know exactly how many bytes it will be in
156 the destination string.
157 The syntax for encoding text strings to binary strings is as simple as
159 decoding:
160 $body = encode('UTF-8', $body);
162 If you needed to know the length of the string in bytes, now's the
164 perfect time for that. Because $body is now a byte string, "length"
165 will report the number of bytes, instead of the number of characters.
166 The number of characters is no longer known, because characters only
167 exist in text strings.
168 my $byte_count = length $body;
170 And if the protocol you're using supports a way of letting the
172 recipient know which character encoding you used, please help the
173 receiving end by using that feature! For example, E-mail and HTTP
174 support MIME headers, so you can use the "Content-Type" header. They
175 can also have "Content-Length" to indicate the number of bytes, which
176 is always a good idea to supply if the number is known.
177 "Content-Type: text/plain; charset=UTF-8",
179 "Content-Length: $byte_count"
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182 Decode everything you receive, encode everything you send out. (If it's
183 text data.)
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186 After reading this document, you ought to read perlunifaq too.
187
189 Thanks to Johan Vromans from Squirrel Consultancy. His UTF-8 rants
190 during the Amsterdam Perl Mongers meetings got me interested and
191 determined to find out how to use character encodings in Perl in ways
192 that don't break easily.
193 Thanks to Gerard Goossen from TTY. His presentation "UTF-8 in the wild"
195 (Dutch Perl Workshop 2006) inspired me to publish my thoughts and write
196 this tutorial.
197 Thanks to the people who asked about this kind of stuff in several Perl
199 IRC channels, and have constantly reminded me that a simpler
200 explanation was needed.
201 Thanks to the people who reviewed this document for me, before it went
203 public. They are: Benjamin Smith, Jan-Pieter Cornet, Johan Vromans,
204 Lukas Mai, Nathan Gray.
205
207 Juerd Waalboer <#####@juerd.nl>
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210 perlunifaq, perlunicode, perluniintro, Encode
211perl v5.16.3 2013-03-04 PERLUNITUT(1)