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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 | |
67 | On a text string, you would do things like: |
68 | |
69 | $text =~ s/foo/bar/; |
70 | if ($string =~ /^\d+$/) { ... } |
71 | $text = ucfirst $text; |
72 | my $character_count = length $text; |
73 | |
74 | The value of a character (C<ord>, C<chr>) is the corresponding Unicode code |
75 | point. |
76 | |
77 | =head3 Binary strings (byte strings) |
78 | |
79 | B<Binary strings>, or B<byte strings> are made of bytes. Here, you don't have |
80 | characters, just bytes. All communication with the outside world (anything |
81 | outside of your current Perl process) is done in binary. |
82 | |
83 | On a binary string, you would do things like: |
84 | |
85 | my (@length_content) = unpack "(V/a)*", $binary; |
86 | $binary =~ s/\x00\x0F/\xFF\xF0/; # for the brave :) |
87 | print {$fh} $binary; |
88 | my $byte_count = length $binary; |
89 | |
90 | =head3 Encoding |
91 | |
92 | B<Encoding> (as a verb) is the conversion from I<text> to I<binary>. To encode, |
93 | you have to supply the target encoding, for example C<iso-8859-1> or C<UTF-8>. |
94 | Some encodings, like the C<iso-8859> ("latin") range, do not support the full |
95 | Unicode standard; characters that can't be represented are lost in the |
96 | conversion. |
97 | |
98 | =head3 Decoding |
99 | |
100 | B<Decoding> is the conversion from I<binary> to I<text>. To decode, you have to |
101 | know what encoding was used during the encoding phase. And most of all, it must |
102 | be something decodable. It doesn't make much sense to decode a PNG image into a |
103 | text string. |
104 | |
105 | =head3 Internal format |
106 | |
107 | Perl has an B<internal format>, an encoding that it uses to encode text strings |
108 | so it can store them in memory. All text strings are in this internal format. |
109 | In fact, text strings are never in any other format! |
110 | |
111 | You shouldn't worry about what this format is, because conversion is |
112 | automatically done when you decode or encode. |
113 | |
114 | =head2 Your new toolkit |
115 | |
116 | Add to your standard heading the following line: |
117 | |
118 | use Encode qw(encode decode); |
119 | |
120 | Or, if you're lazy, just: |
121 | |
122 | use Encode; |
123 | |
124 | =head2 I/O flow (the actual 5 minute tutorial) |
125 | |
126 | The typical input/output flow of a program is: |
127 | |
128 | 1. Receive and decode |
129 | 2. Process |
130 | 3. Encode and output |
131 | |
132 | If your input is binary, and is supposed to remain binary, you shouldn't decode |
133 | it to a text string, of course. But in all other cases, you should decode it. |
134 | |
135 | Decoding can't happen reliably if you don't know how the data was encoded. If |
136 | you get to choose, it's a good idea to standardize on UTF-8. |
137 | |
138 | my $foo = decode('UTF-8', get 'http://example.com/'); |
139 | my $bar = decode('ISO-8859-1', readline STDIN); |
140 | my $xyzzy = decode('Windows-1251', $cgi->param('foo')); |
141 | |
142 | Processing happens as you knew before. The only difference is that you're now |
143 | using characters instead of bytes. That's very useful if you use things like |
144 | C<substr>, or C<length>. |
145 | |
146 | It's important to realize that there are no bytes in a text string. Of course, |
147 | Perl has its internal encoding to store the string in memory, but ignore that. |
148 | If you have to do anything with the number of bytes, it's probably best to move |
149 | that part to step 3, just after you've encoded the string. Then you know |
150 | exactly how many bytes it will be in the destination string. |
151 | |
152 | The syntax for encoding text strings to binary strings is as simple as decoding: |
153 | |
154 | $body = encode('UTF-8', $body); |
155 | |
156 | If you needed to know the length of the string in bytes, now's the perfect time |
157 | for that. Because C<$body> is now a byte string, C<length> will report the |
158 | number of bytes, instead of the number of characters. The number of |
159 | characters is no longer known, because characters only exist in text strings. |
160 | |
161 | my $byte_count = length $body; |
162 | |
163 | And if the protocol you're using supports a way of letting the recipient know |
164 | which character encoding you used, please help the receiving end by using that |
165 | feature! For example, E-mail and HTTP support MIME headers, so you can use the |
166 | C<Content-Type> header. They can also have C<Content-Length> to indicate the |
167 | number of I<bytes>, which is always a good idea to supply if the number is |
168 | known. |
169 | |
170 | "Content-Type: text/plain; charset=UTF-8", |
171 | "Content-Length: $byte_count" |
172 | |
173 | =head2 Q and A |
174 | |
175 | =head3 This isn't really a Unicode tutorial, is it? |
176 | |
177 | No, Perl has an abstracted interface for all supported character encodings, so |
178 | this is actually a generic C<Encode> tutorial. But many people think that |
179 | Unicode is special and magical, and I didn't want to disappoint them, so I |
180 | decided to call this document a Unicode tutorial. |
181 | |
182 | =head3 What about binary data, like images? |
183 | |
184 | Well, apart from a bare C<binmode $fh>, you shouldn't treat them specially. |
185 | (The binmode is needed because otherwise Perl may convert line endings on Win32 |
186 | systems.) |
187 | |
188 | Be careful, though, to never combine text strings with binary strings. If you |
189 | need text in a binary stream, encode your text strings first using the |
190 | appropriate encoding, then join them with binary strings. See also: "What if I |
191 | don't encode?". |
192 | |
193 | =head3 What about the UTF-8 flag? |
194 | |
195 | Please, unless you're hacking the internals, or debugging weirdness, don't |
196 | think about the UTF-8 flag at all. That means that you very probably shouldn't |
197 | use C<is_utf8>, C<_utf8_on> or C<_utf8_off> at all. |
198 | |
199 | Perl's internal format happens to be UTF-8. Unfortunately, Perl can't keep a |
200 | secret, so everyone knows about this. That is the source of much confusion. |
201 | It's better to pretend that the internal format is some unknown encoding, |
202 | and that you always have to encode and decode explicitly. |
203 | |
204 | =head3 When should I decode or encode? |
205 | |
206 | Whenever you're communicating with anything that is external to your perl |
207 | process, like a database, a text file, a socket, or another program. Even if |
208 | the thing you're communicating with is also written in Perl. |
209 | |
210 | =head3 What if I don't decode? |
211 | |
212 | Whenever your encoded, binary string is used together with a text string, Perl |
213 | will assume that your binary string was encoded with ISO-8859-1, also known as |
214 | latin-1. If it wasn't latin-1, then your data is unpleasantly converted. For |
215 | example, if it was UTF-8, the individual bytes of multibyte characters are seen |
216 | as separate characters, and then again converted to UTF-8. Such double encoding |
217 | can be compared to double HTML encoding (C<&gt;>), or double URI encoding |
218 | (C<%253E>). |
219 | |
220 | This silent implicit decoding is known as "upgrading". That may sound |
221 | positive, but it's best to avoid it. |
222 | |
223 | =head3 What if I don't encode? |
224 | |
225 | Your text string will be sent using the bytes in Perl's internal format. In |
226 | some cases, Perl will warn you that you're doing something wrong, with a |
227 | friendly warning: |
228 | |
229 | Wide character in print at example.pl line 2. |
230 | |
231 | Because the internal format is often UTF-8, these bugs are hard to spot, |
232 | because UTF-8 is usually the encoding you wanted! But don't be lazy, and don't |
233 | use the fact that Perl's internal format is UTF-8 to your advantage. Encode |
234 | explicitly to avoid weird bugs, and to show to maintenance programmers that you |
235 | thought this through. |
236 | |
237 | =head3 Is there a way to automatically decode or encode? |
238 | |
239 | If all data that comes from a certain handle is encoded in exactly the same |
240 | way, you can tell the PerlIO system to automatically decode everything, with |
241 | the C<encoding> layer. If you do this, you can't accidentally forget to decode |
242 | or encode anymore, on things that use the layered handle. |
243 | |
244 | You can provide this layer when C<open>ing the file: |
245 | |
246 | open my $fh, '>:encoding(UTF-8)', $filename; # auto encoding on write |
247 | open my $fh, '<:encoding(UTF-8)', $filename; # auto decoding on read |
248 | |
249 | Or if you already have an open filehandle: |
250 | |
251 | binmode $fh, ':encoding(UTF-8)'; |
252 | |
253 | Some database drivers for DBI can also automatically encode and decode, but |
254 | that is typically limited to the UTF-8 encoding, because they cheat. |
255 | |
256 | =head3 Cheat?! Tell me, how can I cheat? |
257 | |
258 | Well, because Perl's internal format is UTF-8, you can just skip the encoding |
259 | or decoding step, and manipulate the UTF-8 flag directly. |
260 | |
261 | Instead of C<:encoding(UTF-8)>, you can simply use C<:utf8>. This is widely |
262 | accepted as good behavior. |
263 | |
264 | Instead of C<decode> and C<encode>, you could use C<_utf8_on> and C<_utf8_off>. |
265 | But this is, contrary to C<:utf8>, considered bad style. |
266 | |
267 | There are some shortcuts for oneliners; see C<-C> in L<perlrun>. |
268 | |
269 | =head3 What if I don't know which encoding was used? |
270 | |
271 | Do whatever you can to find out, and if you have to: guess. (Don't forget to |
272 | document your guess with a comment.) |
273 | |
274 | You could open the document in a web browser, and change the character set or |
275 | character encoding until you can visually confirm that all characters look the |
276 | way they should. |
277 | |
278 | There is no way to reliably detect the encoding automatically, so if people |
279 | keep sending you data without charset indication, you may have to educate them. |
280 | |
281 | =head3 Can I use Unicode in my Perl sources? |
282 | |
283 | Yes, you can! If your sources are UTF-8 encoded, you can indicate that with the |
284 | C<use utf8> pragma. |
285 | |
286 | use utf8; |
287 | |
288 | This doesn't do anything to your input, or to your output. It only influences |
289 | the way your sources are read. You can use Unicode in string literals, in |
290 | identifiers (but they still have to be "word characters" according to C<\w>), |
291 | and even in custom delimiters. |
292 | |
293 | =head3 Data::Dumper doesn't restore the UTF-8 flag; is it broken? |
294 | |
295 | No, Data::Dumper's Unicode abilities are as they should be. There have been |
296 | some complaints that it should restore the UTF-8 flag when the data is read |
297 | again with C<eval>. However, you should really not look at the flag, and |
298 | nothing indicates that Data::Dumper should break this rule. |
299 | |
300 | Here's what happens: when Perl reads in a string literal, it sticks to 8 bit |
301 | encoding as long as it can. (But perhaps originally it was internally encoded |
302 | as UTF-8, when you dumped it.) When it has to give that up because other |
303 | characters are added to the text string, it silently upgrades the string to |
304 | UTF-8. |
305 | |
306 | If you properly encode your strings for output, none of this is of your |
307 | concern, and you can just C<eval> dumped data as always. |
308 | |
309 | =head3 How can I determine if a string is a text string or a binary string? |
310 | |
311 | You can't. Some use the UTF-8 flag for this, but that's misuse, and makes well |
312 | behaved modules like Data::Dumper look bad. The flag is useless for this |
313 | purpose, because it's off when an 8 bit encoding (by default ISO-8859-1) is |
314 | used to store the string. |
315 | |
316 | This is something you, the programmer, has to keep track of; sorry. You could |
317 | consider adopting a kind of "Hungarian notation" to help with this. |
318 | |
319 | =head3 How do I convert from encoding FOO to encoding BAR? |
320 | |
321 | By first converting the FOO-encoded byte string to a text string, and then the |
322 | text string to a BAR-encoded byte string: |
323 | |
324 | my $text_string = decode('FOO', $foo_string); |
325 | my $bar_string = encode('BAR', $text_string); |
326 | |
327 | or by skipping the text string part, and going directly from one binary |
328 | encoding to the other: |
329 | |
330 | use Encode qw(from_to); |
331 | from_to($string, 'FOO', 'BAR'); # changes contents of $string |
332 | |
333 | or by letting automatic decoding and encoding do all the work: |
334 | |
335 | open my $foofh, '<:encoding(FOO)', 'example.foo.txt'; |
336 | open my $barfh, '>:encoding(BAR)', 'example.bar.txt'; |
337 | print { $barfh } $_ while <$foofh>; |
338 | |
339 | =head3 What about the C<use bytes> pragma? |
340 | |
341 | Don't use it. It makes no sense to deal with bytes in a text string, and it |
342 | makes no sense to deal with characters in a byte string. Do the proper |
343 | conversions (by decoding/encoding), and things will work out well: you get |
344 | character counts for decoded data, and byte counts for encoded data. |
345 | |
346 | C<use bytes> is usually a failed attempt to do something useful. Just forget |
347 | about it. |
348 | |
349 | =head3 What are C<decode_utf8> and C<encode_utf8>? |
350 | |
351 | These are alternate syntaxes for C<decode('utf8', ...)> and C<encode('utf8', |
352 | ...)>. |
353 | |
354 | =head3 What's the difference between C<UTF-8> and C<utf8>? |
355 | |
356 | C<UTF-8> is the official standard. C<utf8> is Perl's way of being liberal in |
357 | what it accepts. If you have to communicate with things that aren't so liberal, |
358 | you may want to consider using C<UTF-8>. If you have to communicate with things |
359 | that are too liberal, you may have to use C<utf8>. The full explanation is in |
360 | L<Encode>. |
361 | |
362 | C<UTF-8> is internally known as C<utf-8-strict>. This tutorial uses UTF-8 |
363 | consistently, even where utf8 is actually used internally, because the |
364 | distinction can be hard to make, and is mostly irrelevant. |
365 | |
366 | Okay, if you insist: the "internal format" is utf8, not UTF-8. (When it's not |
367 | some other encoding.) |
368 | |
369 | =head3 I lost track; what encoding is the internal format really? |
370 | |
371 | It's good that you lost track, because you shouldn't depend on the internal |
372 | format being any specific encoding. But since you asked: by default, the |
373 | internal format is either ISO-8859-1 (latin-1), or utf8, depending on the |
374 | history of the string. |
375 | |
376 | Perl knows how it stored the string internally, and will use that knowledge |
377 | when you C<encode>. In other words: don't try to find out what the internal |
378 | encoding for a certain string is, but instead just encode it into the encoding |
379 | that you want. |
380 | |
381 | =head3 What character encodings does Perl support? |
382 | |
383 | To find out which character encodings your Perl supports, run: |
384 | |
385 | perl -MEncode -le "print for Encode->encodings(':all')" |
386 | |
387 | =head3 Which version of perl should I use? |
388 | |
389 | Well, if you can, upgrade to the most recent, but certainly C<5.8.1> or newer. |
390 | This tutorial is based on the status quo as of C<5.8.7>. |
391 | |
392 | You should also check your modules, and upgrade them if necessary. For example, |
393 | HTML::Entities requires version >= 1.32 to function correctly, even though the |
394 | changelog is silent about this. |
395 | |
396 | =head1 SUMMARY |
397 | |
398 | Decode everything you receive, encode everything you send out. (If it's text |
399 | data.) |
400 | |
401 | =head1 ACKNOWLEDGEMENTS |
402 | |
403 | Thanks to Johan Vromans from Squirrel Consultancy. His UTF-8 rants during the |
404 | Amsterdam Perl Mongers meetings got me interested and determined to find out |
405 | how to use character encodings in Perl in ways that don't break easily. |
406 | |
407 | Thanks to Gerard Goossen from TTY. His presentation "UTF-8 in the wild" (Dutch |
408 | Perl Workshop 2006) inspired me to publish my thoughts and write this tutorial. |
409 | |
410 | Thanks to the people who asked about this kind of stuff in several Perl IRC |
411 | channels, and have constantly reminded me that a simpler explanation was |
412 | needed. |
413 | |
414 | Thanks to the people who reviewed this document for me, before it went public. |
415 | They are: Benjamin Smith, Jan-Pieter Cornet, Johan Vromans, Lukas Mai, Nathan |
416 | Gray. |
417 | |
418 | =head1 AUTHOR |
419 | |
420 | Juerd Waalboer <juerd@cpan.org> |
421 | |
422 | =head1 SEE ALSO |
423 | |
424 | L<perlunicode>, L<perluniintro>, L<Encode> |
425 | |