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1=head1 NAME
2
3perlunifaq - Perl Unicode FAQ
4
5=head1 DESCRIPTION
6
7This is a list of questions and answers about Unicode in Perl, intended to be
8read after L<perlunitut>.
9
10=head2 perlunitut isn't really a Unicode tutorial, is it?
11
12No, and this isn't really a Unicode FAQ.
13
14Perl has an abstracted interface for all supported character encodings, so they
15is actually a generic C<Encode> tutorial and C<Encode> FAQ. But many people
16think that Unicode is special and magical, and I didn't want to disappoint
17them, so I decided to call the document a Unicode tutorial.
18
19=head2 What about binary data, like images?
20
21Well, 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
23systems.)
24
25Be careful, though, to never combine text strings with binary strings. If you
26need text in a binary stream, encode your text strings first using the
27appropriate encoding, then join them with binary strings. See also: "What if I
28don't encode?".
29
30=head2 What about the UTF8 flag?
31
32Please, unless you're hacking the internals, or debugging weirdness, don't
33think about the UTF8 flag at all. That means that you very probably shouldn't
34use C<is_utf8>, C<_utf8_on> or C<_utf8_off> at all.
35
36Perl's internal format happens to be UTF-8. Unfortunately, Perl can't keep a
37secret, so everyone knows about this. That is the source of much confusion.
38It's better to pretend that the internal format is some unknown encoding,
39and that you always have to encode and decode explicitly.
40
41=head2 When should I decode or encode?
42
43Whenever you're communicating with anything that is external to your perl
44process, like a database, a text file, a socket, or another program. Even if
45the thing you're communicating with is also written in Perl.
46
47=head2 What if I don't decode?
48
49Whenever your encoded, binary string is used together with a text string, Perl
50will assume that your binary string was encoded with ISO-8859-1, also known as
51latin-1. If it wasn't latin-1, then your data is unpleasantly converted. For
52example, if it was UTF-8, the individual bytes of multibyte characters are seen
53as separate characters, and then again converted to UTF-8. Such double encoding
54can be compared to double HTML encoding (C<&amp;gt;>), or double URI encoding
55(C<%253E>).
56
57This silent implicit decoding is known as "upgrading". That may sound
58positive, but it's best to avoid it.
59
60=head2 What if I don't encode?
61
62Your text string will be sent using the bytes in Perl's internal format. In
63some cases, Perl will warn you that you're doing something wrong, with a
64friendly warning:
65
66 Wide character in print at example.pl line 2.
67
68Because the internal format is often UTF-8, these bugs are hard to spot,
69because UTF-8 is usually the encoding you wanted! But don't be lazy, and don't
70use the fact that Perl's internal format is UTF-8 to your advantage. Encode
71explicitly to avoid weird bugs, and to show to maintenance programmers that you
72thought this through.
73
74=head2 Is there a way to automatically decode or encode?
75
76If all data that comes from a certain handle is encoded in exactly the same
77way, you can tell the PerlIO system to automatically decode everything, with
78the C<encoding> layer. If you do this, you can't accidentally forget to decode
79or encode anymore, on things that use the layered handle.
80
81You 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
86Or if you already have an open filehandle:
87
88 binmode $fh, ':encoding(UTF-8)';
89
90Some database drivers for DBI can also automatically encode and decode, but
91that is typically limited to the UTF-8 encoding, because they cheat.
92
93=head2 Cheat?! Tell me, how can I cheat?
94
95Well, because Perl's internal format is UTF-8, you can just skip the encoding
96or decoding step, and manipulate the UTF8 flag directly.
97
98Instead of C<:encoding(UTF-8)>, you can simply use C<:utf8>. This is widely
99accepted as good behavior when you're writing, but it can be dangerous when
100reading, because it causes internal inconsistency when you have invalid byte
101sequences.
102
103Instead of C<decode> and C<encode>, you could use C<_utf8_on> and C<_utf8_off>,
104but this is considered bad style. Especially C<_utf8_on> can be dangerous, for
105the same reason that C<:utf8> can.
106
107There 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
111Do whatever you can to find out, and if you have to: guess. (Don't forget to
112document your guess with a comment.)
113
114You could open the document in a web browser, and change the character set or
115character encoding until you can visually confirm that all characters look the
116way they should.
117
118There is no way to reliably detect the encoding automatically, so if people
119keep sending you data without charset indication, you may have to educate them.
120
121=head2 Can I use Unicode in my Perl sources?
122
123Yes, you can! If your sources are UTF-8 encoded, you can indicate that with the
124C<use utf8> pragma.
125
126 use utf8;
127
128This doesn't do anything to your input, or to your output. It only influences
129the way your sources are read. You can use Unicode in string literals, in
130identifiers (but they still have to be "word characters" according to C<\w>),
131and even in custom delimiters.
132
133=head2 Data::Dumper doesn't restore the UTF8 flag; is it broken?
134
135No, Data::Dumper's Unicode abilities are as they should be. There have been
136some complaints that it should restore the UTF8 flag when the data is read
137again with C<eval>. However, you should really not look at the flag, and
138nothing indicates that Data::Dumper should break this rule.
139
140Here's what happens: when Perl reads in a string literal, it sticks to 8 bit
141encoding as long as it can. (But perhaps originally it was internally encoded
142as UTF-8, when you dumped it.) When it has to give that up because other
143characters are added to the text string, it silently upgrades the string to
144UTF-8.
145
146If you properly encode your strings for output, none of this is of your
147concern, 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
151You can't. Some use the UTF8 flag for this, but that's misuse, and makes well
152behaved modules like Data::Dumper look bad. The flag is useless for this
153purpose, because it's off when an 8 bit encoding (by default ISO-8859-1) is
154used to store the string.
155
156This is something you, the programmer, has to keep track of; sorry. You could
157consider adopting a kind of "Hungarian notation" to help with this.
158
159=head2 How do I convert from encoding FOO to encoding BAR?
160
161By first converting the FOO-encoded byte string to a text string, and then the
162text string to a BAR-encoded byte string:
163
164 my $text_string = decode('FOO', $foo_string);
165 my $bar_string = encode('BAR', $text_string);
166
167or by skipping the text string part, and going directly from one binary
168encoding to the other:
169
170 use Encode qw(from_to);
171 from_to($string, 'FOO', 'BAR'); # changes contents of $string
172
173or 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
181Don't use it. It makes no sense to deal with bytes in a text string, and it
182makes no sense to deal with characters in a byte string. Do the proper
183conversions (by decoding/encoding), and things will work out well: you get
184character counts for decoded data, and byte counts for encoded data.
185
186C<use bytes> is usually a failed attempt to do something useful. Just forget
187about it.
188
189=head2 What are C<decode_utf8> and C<encode_utf8>?
190
191These 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
196C<UTF-8> is the official standard. C<utf8> is Perl's way of being liberal in
197what it accepts. If you have to communicate with things that aren't so liberal,
198you may want to consider using C<UTF-8>. If you have to communicate with things
199that are too liberal, you may have to use C<utf8>. The full explanation is in
200L<Encode>.
201
202C<UTF-8> is internally known as C<utf-8-strict>. The tutorial uses UTF-8
203consistently, even where utf8 is actually used internally, because the
204distinction can be hard to make, and is mostly irrelevant.
205
206For example, utf8 can be used for code points that don't exist in Unicode, like
2079999999, but if you encode that to UTF-8, you get a substitution character (by
208default; see L<Encode/"Handling Malformed Data"> for more ways of dealing with
209this.)
210
211Okay, if you insist: the "internal format" is utf8, not UTF-8. (When it's not
212some other encoding.)
213
214=head2 I lost track; what encoding is the internal format really?
215
216It's good that you lost track, because you shouldn't depend on the internal
217format being any specific encoding. But since you asked: by default, the
218internal format is either ISO-8859-1 (latin-1), or utf8, depending on the
219history of the string. On EBCDIC platforms, this may be different even.
220
221Perl knows how it stored the string internally, and will use that knowledge
222when you C<encode>. In other words: don't try to find out what the internal
223encoding for a certain string is, but instead just encode it into the encoding
224that you want.
225
226=head2 What character encodings does Perl support?
227
228To 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
234Well, if you can, upgrade to the most recent, but certainly C<5.8.1> or newer.
235The tutorial and FAQ are based on the status quo as of C<5.8.8>.
236
237You should also check your modules, and upgrade them if necessary. For example,
238HTML::Entities requires version >= 1.32 to function correctly, even though the
239changelog is silent about this.
240
241=head1 AUTHOR
242
243Juerd Waalboer <juerd@cpan.org>
244
245=head1 SEE ALSO
246
247L<perlunicode>, L<perluniintro>, L<Encode>
248