| 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>. (But in practice, the |
| 41 | distinction between code point and character is blurred, so the terms often |
| 42 | are used interchangeably.) |
| 43 | |
| 44 | There are many, many code points, but computers work with bytes, and a byte has |
| 45 | room for only 256 values. Unicode has many more characters, so you need a |
| 46 | method to make these accessible. |
| 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 |
| 59 | only one byte per character. All other characters are encoded as two or more |
| 60 | (up to six) bytes using a complex scheme. Fortunately, Perl handles this for |
| 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 | Text strings are also called B<Unicode strings>, because in Perl, every text |
| 70 | string is a Unicode string. |
| 71 | |
| 72 | On a text string, you would do things like: |
| 73 | |
| 74 | $text =~ s/foo/bar/; |
| 75 | if ($string =~ /^\d+$/) { ... } |
| 76 | $text = ucfirst $text; |
| 77 | my $character_count = length $text; |
| 78 | |
| 79 | The value of a character (C<ord>, C<chr>) is the corresponding Unicode code |
| 80 | point. |
| 81 | |
| 82 | =head3 Binary strings (byte strings) |
| 83 | |
| 84 | B<Binary strings>, or B<byte strings> are made of bytes. Here, you don't have |
| 85 | characters, just bytes. All communication with the outside world (anything |
| 86 | outside of your current Perl process) is done in binary. |
| 87 | |
| 88 | On a binary string, you would do things like: |
| 89 | |
| 90 | 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 | |
| 95 | =head3 Encoding |
| 96 | |
| 97 | B<Encoding> (as a verb) is the conversion from I<text> to I<binary>. To encode, |
| 98 | you have to supply the target encoding, for example C<iso-8859-1> or C<UTF-8>. |
| 99 | Some encodings, like the C<iso-8859> ("latin") range, do not support the full |
| 100 | Unicode standard; characters that can't be represented are lost in the |
| 101 | conversion. |
| 102 | |
| 103 | =head3 Decoding |
| 104 | |
| 105 | B<Decoding> is the conversion from I<binary> to I<text>. To decode, you have to |
| 106 | know what encoding was used during the encoding phase. And most of all, it must |
| 107 | be something decodable. It doesn't make much sense to decode a PNG image into a |
| 108 | text string. |
| 109 | |
| 110 | =head3 Internal format |
| 111 | |
| 112 | Perl has an B<internal format>, an encoding that it uses to encode text strings |
| 113 | so it can store them in memory. All text strings are in this internal format. |
| 114 | In fact, text strings are never in any other format! |
| 115 | |
| 116 | You shouldn't worry about what this format is, because conversion is |
| 117 | automatically done when you decode or encode. |
| 118 | |
| 119 | =head2 Your new toolkit |
| 120 | |
| 121 | Add to your standard heading the following line: |
| 122 | |
| 123 | use Encode qw(encode decode); |
| 124 | |
| 125 | Or, if you're lazy, just: |
| 126 | |
| 127 | use Encode; |
| 128 | |
| 129 | =head2 I/O flow (the actual 5 minute tutorial) |
| 130 | |
| 131 | The typical input/output flow of a program is: |
| 132 | |
| 133 | 1. Receive and decode |
| 134 | 2. Process |
| 135 | 3. Encode and output |
| 136 | |
| 137 | If your input is binary, and is supposed to remain binary, you shouldn't decode |
| 138 | it to a text string, of course. But in all other cases, you should decode it. |
| 139 | |
| 140 | Decoding can't happen reliably if you don't know how the data was encoded. If |
| 141 | you get to choose, it's a good idea to standardize on UTF-8. |
| 142 | |
| 143 | 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 | |
| 147 | Processing happens as you knew before. The only difference is that you're now |
| 148 | using characters instead of bytes. That's very useful if you use things like |
| 149 | C<substr>, or C<length>. |
| 150 | |
| 151 | It's important to realize that there are no bytes in a text string. Of course, |
| 152 | Perl has its internal encoding to store the string in memory, but ignore that. |
| 153 | If you have to do anything with the number of bytes, it's probably best to move |
| 154 | that part to step 3, just after you've encoded the string. Then you know |
| 155 | exactly how many bytes it will be in the destination string. |
| 156 | |
| 157 | The syntax for encoding text strings to binary strings is as simple as decoding: |
| 158 | |
| 159 | $body = encode('UTF-8', $body); |
| 160 | |
| 161 | If you needed to know the length of the string in bytes, now's the perfect time |
| 162 | for that. Because C<$body> is now a byte string, C<length> will report the |
| 163 | number of bytes, instead of the number of characters. The number of |
| 164 | characters is no longer known, because characters only exist in text strings. |
| 165 | |
| 166 | my $byte_count = length $body; |
| 167 | |
| 168 | And if the protocol you're using supports a way of letting the recipient know |
| 169 | which character encoding you used, please help the receiving end by using that |
| 170 | feature! For example, E-mail and HTTP support MIME headers, so you can use the |
| 171 | C<Content-Type> header. They can also have C<Content-Length> to indicate the |
| 172 | number of I<bytes>, which is always a good idea to supply if the number is |
| 173 | known. |
| 174 | |
| 175 | "Content-Type: text/plain; charset=UTF-8", |
| 176 | "Content-Length: $byte_count" |
| 177 | |
| 178 | =head1 SUMMARY |
| 179 | |
| 180 | Decode everything you receive, encode everything you send out. (If it's text |
| 181 | data.) |
| 182 | |
| 183 | =head1 Q and A (or FAQ) |
| 184 | |
| 185 | After reading this document, you ought to read L<perlunifaq> too. |
| 186 | |
| 187 | =head1 ACKNOWLEDGEMENTS |
| 188 | |
| 189 | Thanks to Johan Vromans from Squirrel Consultancy. His UTF-8 rants during the |
| 190 | Amsterdam Perl Mongers meetings got me interested and determined to find out |
| 191 | how to use character encodings in Perl in ways that don't break easily. |
| 192 | |
| 193 | Thanks to Gerard Goossen from TTY. His presentation "UTF-8 in the wild" (Dutch |
| 194 | Perl Workshop 2006) inspired me to publish my thoughts and write this tutorial. |
| 195 | |
| 196 | Thanks to the people who asked about this kind of stuff in several Perl IRC |
| 197 | channels, and have constantly reminded me that a simpler explanation was |
| 198 | needed. |
| 199 | |
| 200 | Thanks to the people who reviewed this document for me, before it went public. |
| 201 | They are: Benjamin Smith, Jan-Pieter Cornet, Johan Vromans, Lukas Mai, Nathan |
| 202 | Gray. |
| 203 | |
| 204 | =head1 AUTHOR |
| 205 | |
| 206 | Juerd Waalboer <#####@juerd.nl> |
| 207 | |
| 208 | =head1 SEE ALSO |
| 209 | |
| 210 | L<perlunifaq>, L<perlunicode>, L<perluniintro>, L<Encode> |
| 211 | |