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1 | =head1 NAME |
2 | |
3 | perlunifaq - Perl Unicode FAQ |
4 | |
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5 | =head1 Q and A |
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6 | |
7 | This is a list of questions and answers about Unicode in Perl, intended to be |
8 | read after L<perlunitut>. |
9 | |
10 | =head2 perlunitut isn't really a Unicode tutorial, is it? |
11 | |
12 | No, and this isn't really a Unicode FAQ. |
13 | |
14 | Perl has an abstracted interface for all supported character encodings, so they |
15 | is actually a generic C<Encode> tutorial and C<Encode> FAQ. But many people |
16 | think that Unicode is special and magical, and I didn't want to disappoint |
17 | them, so I decided to call the document a Unicode tutorial. |
18 | |
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19 | =head2 What character encodings does Perl support? |
20 | |
21 | To find out which character encodings your Perl supports, run: |
22 | |
23 | perl -MEncode -le "print for Encode->encodings(':all')" |
24 | |
25 | =head2 Which version of perl should I use? |
26 | |
27 | Well, if you can, upgrade to the most recent, but certainly C<5.8.1> or newer. |
28 | The tutorial and FAQ are based on the status quo as of C<5.8.8>. |
29 | |
30 | You should also check your modules, and upgrade them if necessary. For example, |
31 | HTML::Entities requires version >= 1.32 to function correctly, even though the |
32 | changelog is silent about this. |
33 | |
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34 | =head2 What about binary data, like images? |
35 | |
36 | Well, apart from a bare C<binmode $fh>, you shouldn't treat them specially. |
37 | (The binmode is needed because otherwise Perl may convert line endings on Win32 |
38 | systems.) |
39 | |
40 | Be careful, though, to never combine text strings with binary strings. If you |
41 | need text in a binary stream, encode your text strings first using the |
42 | appropriate encoding, then join them with binary strings. See also: "What if I |
43 | don't encode?". |
44 | |
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45 | =head2 When should I decode or encode? |
46 | |
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47 | Whenever you're communicating text with anything that is external to your perl |
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48 | process, like a database, a text file, a socket, or another program. Even if |
49 | the thing you're communicating with is also written in Perl. |
50 | |
51 | =head2 What if I don't decode? |
52 | |
53 | Whenever your encoded, binary string is used together with a text string, Perl |
54 | will assume that your binary string was encoded with ISO-8859-1, also known as |
55 | latin-1. If it wasn't latin-1, then your data is unpleasantly converted. For |
56 | example, if it was UTF-8, the individual bytes of multibyte characters are seen |
57 | as separate characters, and then again converted to UTF-8. Such double encoding |
58 | can be compared to double HTML encoding (C<&gt;>), or double URI encoding |
59 | (C<%253E>). |
60 | |
61 | This silent implicit decoding is known as "upgrading". That may sound |
62 | positive, but it's best to avoid it. |
63 | |
64 | =head2 What if I don't encode? |
65 | |
66 | Your text string will be sent using the bytes in Perl's internal format. In |
67 | some cases, Perl will warn you that you're doing something wrong, with a |
68 | friendly warning: |
69 | |
70 | Wide character in print at example.pl line 2. |
71 | |
72 | Because the internal format is often UTF-8, these bugs are hard to spot, |
73 | because UTF-8 is usually the encoding you wanted! But don't be lazy, and don't |
74 | use the fact that Perl's internal format is UTF-8 to your advantage. Encode |
75 | explicitly to avoid weird bugs, and to show to maintenance programmers that you |
76 | thought this through. |
77 | |
78 | =head2 Is there a way to automatically decode or encode? |
79 | |
80 | If all data that comes from a certain handle is encoded in exactly the same |
81 | way, you can tell the PerlIO system to automatically decode everything, with |
82 | the C<encoding> layer. If you do this, you can't accidentally forget to decode |
83 | or encode anymore, on things that use the layered handle. |
84 | |
85 | You can provide this layer when C<open>ing the file: |
86 | |
87 | open my $fh, '>:encoding(UTF-8)', $filename; # auto encoding on write |
88 | open my $fh, '<:encoding(UTF-8)', $filename; # auto decoding on read |
89 | |
90 | Or if you already have an open filehandle: |
91 | |
92 | binmode $fh, ':encoding(UTF-8)'; |
93 | |
94 | Some database drivers for DBI can also automatically encode and decode, but |
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95 | that is sometimes limited to the UTF-8 encoding. |
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96 | |
97 | =head2 What if I don't know which encoding was used? |
98 | |
99 | Do whatever you can to find out, and if you have to: guess. (Don't forget to |
100 | document your guess with a comment.) |
101 | |
102 | You could open the document in a web browser, and change the character set or |
103 | character encoding until you can visually confirm that all characters look the |
104 | way they should. |
105 | |
106 | There is no way to reliably detect the encoding automatically, so if people |
107 | keep sending you data without charset indication, you may have to educate them. |
108 | |
109 | =head2 Can I use Unicode in my Perl sources? |
110 | |
111 | Yes, you can! If your sources are UTF-8 encoded, you can indicate that with the |
112 | C<use utf8> pragma. |
113 | |
114 | use utf8; |
115 | |
116 | This doesn't do anything to your input, or to your output. It only influences |
117 | the way your sources are read. You can use Unicode in string literals, in |
118 | identifiers (but they still have to be "word characters" according to C<\w>), |
119 | and even in custom delimiters. |
120 | |
121 | =head2 Data::Dumper doesn't restore the UTF8 flag; is it broken? |
122 | |
123 | No, Data::Dumper's Unicode abilities are as they should be. There have been |
124 | some complaints that it should restore the UTF8 flag when the data is read |
125 | again with C<eval>. However, you should really not look at the flag, and |
126 | nothing indicates that Data::Dumper should break this rule. |
127 | |
128 | Here's what happens: when Perl reads in a string literal, it sticks to 8 bit |
129 | encoding as long as it can. (But perhaps originally it was internally encoded |
130 | as UTF-8, when you dumped it.) When it has to give that up because other |
131 | characters are added to the text string, it silently upgrades the string to |
132 | UTF-8. |
133 | |
134 | If you properly encode your strings for output, none of this is of your |
135 | concern, and you can just C<eval> dumped data as always. |
136 | |
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137 | =head2 Why do regex character classes sometimes match only in the ASCII range? |
138 | |
139 | =head2 Why do some characters not uppercase or lowercase correctly? |
140 | |
141 | It seemed like a good idea at the time, to keep the semantics the same for |
142 | standard strings, when Perl got Unicode support. While it might be repaired |
143 | in the future, we now have to deal with the fact that Perl treats equal |
144 | strings differently, depending on the internal state. |
145 | |
146 | Affected are C<uc>, C<lc>, C<ucfirst>, C<lcfirst>, C<\U>, C<\L>, C<\u>, C<\l>, |
147 | C<\d>, C<\s>, C<\w>, C<\D>, C<\S>, C<\W>, C</.../i>, C<(?i:...)>, |
148 | C</[[:posix:]]/>. |
149 | |
150 | To force Unicode semantics, you can upgrade the internal representation to |
151 | by doing C<utf8::upgrade($string)>. This does not change strings that were |
152 | already upgraded. |
153 | |
154 | For a more detailed discussion, see L<Unicode::Semantics> on CPAN. |
155 | |
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156 | =head2 How can I determine if a string is a text string or a binary string? |
157 | |
158 | You can't. Some use the UTF8 flag for this, but that's misuse, and makes well |
159 | behaved modules like Data::Dumper look bad. The flag is useless for this |
160 | purpose, because it's off when an 8 bit encoding (by default ISO-8859-1) is |
161 | used to store the string. |
162 | |
163 | This is something you, the programmer, has to keep track of; sorry. You could |
164 | consider adopting a kind of "Hungarian notation" to help with this. |
165 | |
166 | =head2 How do I convert from encoding FOO to encoding BAR? |
167 | |
168 | By first converting the FOO-encoded byte string to a text string, and then the |
169 | text string to a BAR-encoded byte string: |
170 | |
171 | my $text_string = decode('FOO', $foo_string); |
172 | my $bar_string = encode('BAR', $text_string); |
173 | |
174 | or by skipping the text string part, and going directly from one binary |
175 | encoding to the other: |
176 | |
177 | use Encode qw(from_to); |
178 | from_to($string, 'FOO', 'BAR'); # changes contents of $string |
179 | |
180 | or by letting automatic decoding and encoding do all the work: |
181 | |
182 | open my $foofh, '<:encoding(FOO)', 'example.foo.txt'; |
183 | open my $barfh, '>:encoding(BAR)', 'example.bar.txt'; |
184 | print { $barfh } $_ while <$foofh>; |
185 | |
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186 | =head2 What are C<decode_utf8> and C<encode_utf8>? |
187 | |
188 | These are alternate syntaxes for C<decode('utf8', ...)> and C<encode('utf8', |
189 | ...)>. |
190 | |
191 | =head2 What is a "wide character"? |
192 | |
193 | This is a term used both for characters with an ordinal value greater than 127, |
194 | characters with an ordinal value greater than 255, or any character occupying |
195 | than one byte, depending on the context. |
196 | |
197 | The Perl warning "Wide character in ..." is caused by a character with an |
198 | ordinal value greater than 255. With no specified encoding layer, Perl tries to |
199 | fit things in ISO-8859-1 for backward compatibility reasons. When it can't, it |
200 | emits this warning (if warnings are enabled), and outputs UTF-8 encoded data |
201 | instead. |
202 | |
203 | To avoid this warning and to avoid having different output encodings in a single |
204 | stream, always specify an encoding explicitly, for example with a PerlIO layer: |
205 | |
206 | binmode STDOUT, ":encoding(UTF-8)"; |
207 | |
208 | =head1 INTERNALS |
209 | |
210 | =head2 What is "the UTF8 flag"? |
211 | |
212 | Please, unless you're hacking the internals, or debugging weirdness, don't |
213 | think about the UTF8 flag at all. That means that you very probably shouldn't |
214 | use C<is_utf8>, C<_utf8_on> or C<_utf8_off> at all. |
215 | |
216 | The UTF8 flag, also called SvUTF8, is an internal flag that indicates that the |
217 | current internal representation is UTF-8. Without the flag, it is assumed to be |
218 | ISO-8859-1. Perl converts between these automatically. |
219 | |
220 | One of Perl's internal formats happens to be UTF-8. Unfortunately, Perl can't |
221 | keep a secret, so everyone knows about this. That is the source of much |
222 | confusion. It's better to pretend that the internal format is some unknown |
223 | encoding, and that you always have to encode and decode explicitly. |
224 | |
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225 | =head2 What about the C<use bytes> pragma? |
226 | |
227 | Don't use it. It makes no sense to deal with bytes in a text string, and it |
228 | makes no sense to deal with characters in a byte string. Do the proper |
229 | conversions (by decoding/encoding), and things will work out well: you get |
230 | character counts for decoded data, and byte counts for encoded data. |
231 | |
232 | C<use bytes> is usually a failed attempt to do something useful. Just forget |
233 | about it. |
234 | |
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235 | =head2 What about the C<use encoding> pragma? |
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236 | |
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237 | Don't use it. Unfortunately, it assumes that the programmer's environment and |
238 | that of the user will use the same encoding. It will use the same encoding for |
239 | the source code and for STDIN and STDOUT. When a program is copied to another |
240 | machine, the source code does not change, but the STDIO environment might. |
241 | |
242 | If you need non-ASCII characters in your source code, make it a UTF-8 encoded |
243 | file and C<use utf8>. |
244 | |
245 | If you need to set the encoding for STDIN, STDOUT, and STDERR, for example |
246 | based on the user's locale, C<use open>. |
247 | |
248 | =head2 What is the difference between C<:encoding> and C<:utf8>? |
249 | |
250 | Because UTF-8 is one of Perl's internal formats, you can often just skip the |
251 | encoding or decoding step, and manipulate the UTF8 flag directly. |
252 | |
253 | Instead of C<:encoding(UTF-8)>, you can simply use C<:utf8>, which skips the |
254 | encoding step if the data was already represented as UTF8 internally. This is |
255 | widely accepted as good behavior when you're writing, but it can be dangerous |
256 | when reading, because it causes internal inconsistency when you have invalid |
257 | byte sequences. Using C<:utf8> for input can sometimes result in security |
258 | breaches, so please use C<:encoding(UTF-8)> instead. |
259 | |
260 | Instead of C<decode> and C<encode>, you could use C<_utf8_on> and C<_utf8_off>, |
261 | but this is considered bad style. Especially C<_utf8_on> can be dangerous, for |
262 | the same reason that C<:utf8> can. |
263 | |
264 | There are some shortcuts for oneliners; see C<-C> in L<perlrun>. |
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265 | |
266 | =head2 What's the difference between C<UTF-8> and C<utf8>? |
267 | |
268 | C<UTF-8> is the official standard. C<utf8> is Perl's way of being liberal in |
269 | what it accepts. If you have to communicate with things that aren't so liberal, |
270 | you may want to consider using C<UTF-8>. If you have to communicate with things |
271 | that are too liberal, you may have to use C<utf8>. The full explanation is in |
272 | L<Encode>. |
273 | |
274 | C<UTF-8> is internally known as C<utf-8-strict>. The tutorial uses UTF-8 |
275 | consistently, even where utf8 is actually used internally, because the |
276 | distinction can be hard to make, and is mostly irrelevant. |
277 | |
278 | For example, utf8 can be used for code points that don't exist in Unicode, like |
279 | 9999999, but if you encode that to UTF-8, you get a substitution character (by |
280 | default; see L<Encode/"Handling Malformed Data"> for more ways of dealing with |
281 | this.) |
282 | |
283 | Okay, if you insist: the "internal format" is utf8, not UTF-8. (When it's not |
284 | some other encoding.) |
285 | |
286 | =head2 I lost track; what encoding is the internal format really? |
287 | |
288 | It's good that you lost track, because you shouldn't depend on the internal |
289 | format being any specific encoding. But since you asked: by default, the |
290 | internal format is either ISO-8859-1 (latin-1), or utf8, depending on the |
291 | history of the string. On EBCDIC platforms, this may be different even. |
292 | |
293 | Perl knows how it stored the string internally, and will use that knowledge |
294 | when you C<encode>. In other words: don't try to find out what the internal |
295 | encoding for a certain string is, but instead just encode it into the encoding |
296 | that you want. |
297 | |
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298 | =head1 AUTHOR |
299 | |
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300 | Juerd Waalboer <#####@juerd.nl> |
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301 | |
302 | =head1 SEE ALSO |
303 | |
304 | L<perlunicode>, L<perluniintro>, L<Encode> |
305 | |