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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 |