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1 | =head1 NAME |
2 | |
3 | perlunifaq - Perl Unicode FAQ |
4 | |
5 | =head1 DESCRIPTION |
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 | |
19 | =head2 What about binary data, like images? |
20 | |
21 | Well, apart from a bare C<binmode $fh>, you shouldn't treat them specially. |
22 | (The binmode is needed because otherwise Perl may convert line endings on Win32 |
23 | systems.) |
24 | |
25 | Be careful, though, to never combine text strings with binary strings. If you |
26 | need text in a binary stream, encode your text strings first using the |
27 | appropriate encoding, then join them with binary strings. See also: "What if I |
28 | don't encode?". |
29 | |
30 | =head2 What about the UTF8 flag? |
31 | |
32 | Please, unless you're hacking the internals, or debugging weirdness, don't |
33 | think about the UTF8 flag at all. That means that you very probably shouldn't |
34 | use C<is_utf8>, C<_utf8_on> or C<_utf8_off> at all. |
35 | |
36 | Perl's internal format happens to be UTF-8. Unfortunately, Perl can't keep a |
37 | secret, so everyone knows about this. That is the source of much confusion. |
38 | It's better to pretend that the internal format is some unknown encoding, |
39 | and that you always have to encode and decode explicitly. |
40 | |
41 | =head2 When should I decode or encode? |
42 | |
43 | Whenever you're communicating with anything that is external to your perl |
44 | process, like a database, a text file, a socket, or another program. Even if |
45 | the thing you're communicating with is also written in Perl. |
46 | |
47 | =head2 What if I don't decode? |
48 | |
49 | Whenever your encoded, binary string is used together with a text string, Perl |
50 | will assume that your binary string was encoded with ISO-8859-1, also known as |
51 | latin-1. If it wasn't latin-1, then your data is unpleasantly converted. For |
52 | example, if it was UTF-8, the individual bytes of multibyte characters are seen |
53 | as separate characters, and then again converted to UTF-8. Such double encoding |
54 | can be compared to double HTML encoding (C<&gt;>), or double URI encoding |
55 | (C<%253E>). |
56 | |
57 | This silent implicit decoding is known as "upgrading". That may sound |
58 | positive, but it's best to avoid it. |
59 | |
60 | =head2 What if I don't encode? |
61 | |
62 | Your text string will be sent using the bytes in Perl's internal format. In |
63 | some cases, Perl will warn you that you're doing something wrong, with a |
64 | friendly warning: |
65 | |
66 | Wide character in print at example.pl line 2. |
67 | |
68 | Because the internal format is often UTF-8, these bugs are hard to spot, |
69 | because UTF-8 is usually the encoding you wanted! But don't be lazy, and don't |
70 | use the fact that Perl's internal format is UTF-8 to your advantage. Encode |
71 | explicitly to avoid weird bugs, and to show to maintenance programmers that you |
72 | thought this through. |
73 | |
74 | =head2 Is there a way to automatically decode or encode? |
75 | |
76 | If all data that comes from a certain handle is encoded in exactly the same |
77 | way, you can tell the PerlIO system to automatically decode everything, with |
78 | the C<encoding> layer. If you do this, you can't accidentally forget to decode |
79 | or encode anymore, on things that use the layered handle. |
80 | |
81 | You can provide this layer when C<open>ing the file: |
82 | |
83 | open my $fh, '>:encoding(UTF-8)', $filename; # auto encoding on write |
84 | open my $fh, '<:encoding(UTF-8)', $filename; # auto decoding on read |
85 | |
86 | Or if you already have an open filehandle: |
87 | |
88 | binmode $fh, ':encoding(UTF-8)'; |
89 | |
90 | Some database drivers for DBI can also automatically encode and decode, but |
91 | that is typically limited to the UTF-8 encoding, because they cheat. |
92 | |
93 | =head2 Cheat?! Tell me, how can I cheat? |
94 | |
95 | Well, because Perl's internal format is UTF-8, you can just skip the encoding |
96 | or decoding step, and manipulate the UTF8 flag directly. |
97 | |
98 | Instead of C<:encoding(UTF-8)>, you can simply use C<:utf8>. This is widely |
99 | accepted as good behavior when you're writing, but it can be dangerous when |
100 | reading, because it causes internal inconsistency when you have invalid byte |
101 | sequences. |
102 | |
103 | Instead of C<decode> and C<encode>, you could use C<_utf8_on> and C<_utf8_off>, |
104 | but this is considered bad style. Especially C<_utf8_on> can be dangerous, for |
105 | the same reason that C<:utf8> can. |
106 | |
107 | There are some shortcuts for oneliners; see C<-C> in L<perlrun>. |
108 | |
109 | =head2 What if I don't know which encoding was used? |
110 | |
111 | Do whatever you can to find out, and if you have to: guess. (Don't forget to |
112 | document your guess with a comment.) |
113 | |
114 | You could open the document in a web browser, and change the character set or |
115 | character encoding until you can visually confirm that all characters look the |
116 | way they should. |
117 | |
118 | There is no way to reliably detect the encoding automatically, so if people |
119 | keep sending you data without charset indication, you may have to educate them. |
120 | |
121 | =head2 Can I use Unicode in my Perl sources? |
122 | |
123 | Yes, you can! If your sources are UTF-8 encoded, you can indicate that with the |
124 | C<use utf8> pragma. |
125 | |
126 | use utf8; |
127 | |
128 | This doesn't do anything to your input, or to your output. It only influences |
129 | the way your sources are read. You can use Unicode in string literals, in |
130 | identifiers (but they still have to be "word characters" according to C<\w>), |
131 | and even in custom delimiters. |
132 | |
133 | =head2 Data::Dumper doesn't restore the UTF8 flag; is it broken? |
134 | |
135 | No, Data::Dumper's Unicode abilities are as they should be. There have been |
136 | some complaints that it should restore the UTF8 flag when the data is read |
137 | again with C<eval>. However, you should really not look at the flag, and |
138 | nothing indicates that Data::Dumper should break this rule. |
139 | |
140 | Here's what happens: when Perl reads in a string literal, it sticks to 8 bit |
141 | encoding as long as it can. (But perhaps originally it was internally encoded |
142 | as UTF-8, when you dumped it.) When it has to give that up because other |
143 | characters are added to the text string, it silently upgrades the string to |
144 | UTF-8. |
145 | |
146 | If you properly encode your strings for output, none of this is of your |
147 | concern, and you can just C<eval> dumped data as always. |
148 | |
149 | =head2 How can I determine if a string is a text string or a binary string? |
150 | |
151 | You can't. Some use the UTF8 flag for this, but that's misuse, and makes well |
152 | behaved modules like Data::Dumper look bad. The flag is useless for this |
153 | purpose, because it's off when an 8 bit encoding (by default ISO-8859-1) is |
154 | used to store the string. |
155 | |
156 | This is something you, the programmer, has to keep track of; sorry. You could |
157 | consider adopting a kind of "Hungarian notation" to help with this. |
158 | |
159 | =head2 How do I convert from encoding FOO to encoding BAR? |
160 | |
161 | By first converting the FOO-encoded byte string to a text string, and then the |
162 | text string to a BAR-encoded byte string: |
163 | |
164 | my $text_string = decode('FOO', $foo_string); |
165 | my $bar_string = encode('BAR', $text_string); |
166 | |
167 | or by skipping the text string part, and going directly from one binary |
168 | encoding to the other: |
169 | |
170 | use Encode qw(from_to); |
171 | from_to($string, 'FOO', 'BAR'); # changes contents of $string |
172 | |
173 | or by letting automatic decoding and encoding do all the work: |
174 | |
175 | open my $foofh, '<:encoding(FOO)', 'example.foo.txt'; |
176 | open my $barfh, '>:encoding(BAR)', 'example.bar.txt'; |
177 | print { $barfh } $_ while <$foofh>; |
178 | |
179 | =head2 What about the C<use bytes> pragma? |
180 | |
181 | Don't use it. It makes no sense to deal with bytes in a text string, and it |
182 | makes no sense to deal with characters in a byte string. Do the proper |
183 | conversions (by decoding/encoding), and things will work out well: you get |
184 | character counts for decoded data, and byte counts for encoded data. |
185 | |
186 | C<use bytes> is usually a failed attempt to do something useful. Just forget |
187 | about it. |
188 | |
189 | =head2 What are C<decode_utf8> and C<encode_utf8>? |
190 | |
191 | These are alternate syntaxes for C<decode('utf8', ...)> and C<encode('utf8', |
192 | ...)>. |
193 | |
194 | =head2 What's the difference between C<UTF-8> and C<utf8>? |
195 | |
196 | C<UTF-8> is the official standard. C<utf8> is Perl's way of being liberal in |
197 | what it accepts. If you have to communicate with things that aren't so liberal, |
198 | you may want to consider using C<UTF-8>. If you have to communicate with things |
199 | that are too liberal, you may have to use C<utf8>. The full explanation is in |
200 | L<Encode>. |
201 | |
202 | C<UTF-8> is internally known as C<utf-8-strict>. The tutorial uses UTF-8 |
203 | consistently, even where utf8 is actually used internally, because the |
204 | distinction can be hard to make, and is mostly irrelevant. |
205 | |
206 | For example, utf8 can be used for code points that don't exist in Unicode, like |
207 | 9999999, but if you encode that to UTF-8, you get a substitution character (by |
208 | default; see L<Encode/"Handling Malformed Data"> for more ways of dealing with |
209 | this.) |
210 | |
211 | Okay, if you insist: the "internal format" is utf8, not UTF-8. (When it's not |
212 | some other encoding.) |
213 | |
214 | =head2 I lost track; what encoding is the internal format really? |
215 | |
216 | It's good that you lost track, because you shouldn't depend on the internal |
217 | format being any specific encoding. But since you asked: by default, the |
218 | internal format is either ISO-8859-1 (latin-1), or utf8, depending on the |
219 | history of the string. On EBCDIC platforms, this may be different even. |
220 | |
221 | Perl knows how it stored the string internally, and will use that knowledge |
222 | when you C<encode>. In other words: don't try to find out what the internal |
223 | encoding for a certain string is, but instead just encode it into the encoding |
224 | that you want. |
225 | |
226 | =head2 What character encodings does Perl support? |
227 | |
228 | To find out which character encodings your Perl supports, run: |
229 | |
230 | perl -MEncode -le "print for Encode->encodings(':all')" |
231 | |
232 | =head2 Which version of perl should I use? |
233 | |
234 | Well, if you can, upgrade to the most recent, but certainly C<5.8.1> or newer. |
235 | The tutorial and FAQ are based on the status quo as of C<5.8.8>. |
236 | |
237 | You should also check your modules, and upgrade them if necessary. For example, |
238 | HTML::Entities requires version >= 1.32 to function correctly, even though the |
239 | changelog is silent about this. |
240 | |
241 | =head1 AUTHOR |
242 | |
243 | Juerd Waalboer <juerd@cpan.org> |
244 | |
245 | =head1 SEE ALSO |
246 | |
247 | L<perlunicode>, L<perluniintro>, L<Encode> |
248 | |