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1=head1 NAME
2
3perlunifaq - Perl Unicode FAQ
4
740d4bb2 5=head1 Q and A
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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
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19=head2 What character encodings does Perl support?
20
21To 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
27Well, if you can, upgrade to the most recent, but certainly C<5.8.1> or newer.
28The tutorial and FAQ are based on the status quo as of C<5.8.8>.
29
30You should also check your modules, and upgrade them if necessary. For example,
31HTML::Entities requires version >= 1.32 to function correctly, even though the
32changelog is silent about this.
33
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34=head2 What about binary data, like images?
35
36Well, 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
38systems.)
39
40Be careful, though, to never combine text strings with binary strings. If you
41need text in a binary stream, encode your text strings first using the
42appropriate encoding, then join them with binary strings. See also: "What if I
43don't encode?".
44
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45=head2 When should I decode or encode?
46
740d4bb2 47Whenever you're communicating text with anything that is external to your perl
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48process, like a database, a text file, a socket, or another program. Even if
49the thing you're communicating with is also written in Perl.
50
51=head2 What if I don't decode?
52
53Whenever your encoded, binary string is used together with a text string, Perl
54will assume that your binary string was encoded with ISO-8859-1, also known as
55latin-1. If it wasn't latin-1, then your data is unpleasantly converted. For
56example, if it was UTF-8, the individual bytes of multibyte characters are seen
57as separate characters, and then again converted to UTF-8. Such double encoding
58can be compared to double HTML encoding (C<&amp;gt;>), or double URI encoding
59(C<%253E>).
60
61This silent implicit decoding is known as "upgrading". That may sound
62positive, but it's best to avoid it.
63
64=head2 What if I don't encode?
65
66Your text string will be sent using the bytes in Perl's internal format. In
67some cases, Perl will warn you that you're doing something wrong, with a
68friendly warning:
69
70 Wide character in print at example.pl line 2.
71
72Because the internal format is often UTF-8, these bugs are hard to spot,
73because UTF-8 is usually the encoding you wanted! But don't be lazy, and don't
74use the fact that Perl's internal format is UTF-8 to your advantage. Encode
75explicitly to avoid weird bugs, and to show to maintenance programmers that you
76thought this through.
77
78=head2 Is there a way to automatically decode or encode?
79
80If all data that comes from a certain handle is encoded in exactly the same
81way, you can tell the PerlIO system to automatically decode everything, with
82the C<encoding> layer. If you do this, you can't accidentally forget to decode
83or encode anymore, on things that use the layered handle.
84
85You 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
90Or if you already have an open filehandle:
91
92 binmode $fh, ':encoding(UTF-8)';
93
94Some database drivers for DBI can also automatically encode and decode, but
740d4bb2 95that 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
99Do whatever you can to find out, and if you have to: guess. (Don't forget to
100document your guess with a comment.)
101
102You could open the document in a web browser, and change the character set or
103character encoding until you can visually confirm that all characters look the
104way they should.
105
106There is no way to reliably detect the encoding automatically, so if people
107keep sending you data without charset indication, you may have to educate them.
108
109=head2 Can I use Unicode in my Perl sources?
110
111Yes, you can! If your sources are UTF-8 encoded, you can indicate that with the
112C<use utf8> pragma.
113
114 use utf8;
115
116This doesn't do anything to your input, or to your output. It only influences
117the way your sources are read. You can use Unicode in string literals, in
118identifiers (but they still have to be "word characters" according to C<\w>),
119and even in custom delimiters.
120
121=head2 Data::Dumper doesn't restore the UTF8 flag; is it broken?
122
123No, Data::Dumper's Unicode abilities are as they should be. There have been
124some complaints that it should restore the UTF8 flag when the data is read
125again with C<eval>. However, you should really not look at the flag, and
126nothing indicates that Data::Dumper should break this rule.
127
128Here's what happens: when Perl reads in a string literal, it sticks to 8 bit
129encoding as long as it can. (But perhaps originally it was internally encoded
130as UTF-8, when you dumped it.) When it has to give that up because other
131characters are added to the text string, it silently upgrades the string to
132UTF-8.
133
134If you properly encode your strings for output, none of this is of your
135concern, 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
141It seemed like a good idea at the time, to keep the semantics the same for
142standard strings, when Perl got Unicode support. While it might be repaired
143in the future, we now have to deal with the fact that Perl treats equal
144strings differently, depending on the internal state.
145
146Affected are C<uc>, C<lc>, C<ucfirst>, C<lcfirst>, C<\U>, C<\L>, C<\u>, C<\l>,
147C<\d>, C<\s>, C<\w>, C<\D>, C<\S>, C<\W>, C</.../i>, C<(?i:...)>,
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148C</[[:posix:]]/>, and C<quotemeta> (though this last should not cause any real
149problems).
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150
151To force Unicode semantics, you can upgrade the internal representation to
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152by doing C<utf8::upgrade($string)>. This can be used
153safely on any string, as it checks and does not change strings that have
154already been upgraded.
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155
156For a more detailed discussion, see L<Unicode::Semantics> on CPAN.
157
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158=head2 How can I determine if a string is a text string or a binary string?
159
160You can't. Some use the UTF8 flag for this, but that's misuse, and makes well
161behaved modules like Data::Dumper look bad. The flag is useless for this
162purpose, because it's off when an 8 bit encoding (by default ISO-8859-1) is
163used to store the string.
164
165This is something you, the programmer, has to keep track of; sorry. You could
166consider adopting a kind of "Hungarian notation" to help with this.
167
168=head2 How do I convert from encoding FOO to encoding BAR?
169
170By first converting the FOO-encoded byte string to a text string, and then the
171text string to a BAR-encoded byte string:
172
173 my $text_string = decode('FOO', $foo_string);
174 my $bar_string = encode('BAR', $text_string);
175
176or by skipping the text string part, and going directly from one binary
177encoding to the other:
178
179 use Encode qw(from_to);
180 from_to($string, 'FOO', 'BAR'); # changes contents of $string
181
182or by letting automatic decoding and encoding do all the work:
183
184 open my $foofh, '<:encoding(FOO)', 'example.foo.txt';
185 open my $barfh, '>:encoding(BAR)', 'example.bar.txt';
186 print { $barfh } $_ while <$foofh>;
187
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188=head2 What are C<decode_utf8> and C<encode_utf8>?
189
190These are alternate syntaxes for C<decode('utf8', ...)> and C<encode('utf8',
191...)>.
192
193=head2 What is a "wide character"?
194
195This is a term used both for characters with an ordinal value greater than 127,
196characters with an ordinal value greater than 255, or any character occupying
197than one byte, depending on the context.
198
199The Perl warning "Wide character in ..." is caused by a character with an
200ordinal value greater than 255. With no specified encoding layer, Perl tries to
201fit things in ISO-8859-1 for backward compatibility reasons. When it can't, it
202emits this warning (if warnings are enabled), and outputs UTF-8 encoded data
203instead.
204
205To avoid this warning and to avoid having different output encodings in a single
206stream, always specify an encoding explicitly, for example with a PerlIO layer:
207
208 binmode STDOUT, ":encoding(UTF-8)";
209
210=head1 INTERNALS
211
212=head2 What is "the UTF8 flag"?
213
214Please, unless you're hacking the internals, or debugging weirdness, don't
215think about the UTF8 flag at all. That means that you very probably shouldn't
216use C<is_utf8>, C<_utf8_on> or C<_utf8_off> at all.
217
218The UTF8 flag, also called SvUTF8, is an internal flag that indicates that the
219current internal representation is UTF-8. Without the flag, it is assumed to be
220ISO-8859-1. Perl converts between these automatically.
221
222One of Perl's internal formats happens to be UTF-8. Unfortunately, Perl can't
223keep a secret, so everyone knows about this. That is the source of much
224confusion. It's better to pretend that the internal format is some unknown
225encoding, and that you always have to encode and decode explicitly.
226
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227=head2 What about the C<use bytes> pragma?
228
229Don't use it. It makes no sense to deal with bytes in a text string, and it
230makes no sense to deal with characters in a byte string. Do the proper
231conversions (by decoding/encoding), and things will work out well: you get
232character counts for decoded data, and byte counts for encoded data.
233
234C<use bytes> is usually a failed attempt to do something useful. Just forget
235about it.
236
740d4bb2 237=head2 What about the C<use encoding> pragma?
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239Don't use it. Unfortunately, it assumes that the programmer's environment and
240that of the user will use the same encoding. It will use the same encoding for
241the source code and for STDIN and STDOUT. When a program is copied to another
242machine, the source code does not change, but the STDIO environment might.
243
244If you need non-ASCII characters in your source code, make it a UTF-8 encoded
245file and C<use utf8>.
246
247If you need to set the encoding for STDIN, STDOUT, and STDERR, for example
248based on the user's locale, C<use open>.
249
250=head2 What is the difference between C<:encoding> and C<:utf8>?
251
252Because UTF-8 is one of Perl's internal formats, you can often just skip the
253encoding or decoding step, and manipulate the UTF8 flag directly.
254
255Instead of C<:encoding(UTF-8)>, you can simply use C<:utf8>, which skips the
256encoding step if the data was already represented as UTF8 internally. This is
257widely accepted as good behavior when you're writing, but it can be dangerous
258when reading, because it causes internal inconsistency when you have invalid
259byte sequences. Using C<:utf8> for input can sometimes result in security
260breaches, so please use C<:encoding(UTF-8)> instead.
261
262Instead of C<decode> and C<encode>, you could use C<_utf8_on> and C<_utf8_off>,
263but this is considered bad style. Especially C<_utf8_on> can be dangerous, for
264the same reason that C<:utf8> can.
265
266There are some shortcuts for oneliners; see C<-C> in L<perlrun>.
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267
268=head2 What's the difference between C<UTF-8> and C<utf8>?
269
270C<UTF-8> is the official standard. C<utf8> is Perl's way of being liberal in
271what it accepts. If you have to communicate with things that aren't so liberal,
272you may want to consider using C<UTF-8>. If you have to communicate with things
273that are too liberal, you may have to use C<utf8>. The full explanation is in
274L<Encode>.
275
276C<UTF-8> is internally known as C<utf-8-strict>. The tutorial uses UTF-8
277consistently, even where utf8 is actually used internally, because the
278distinction can be hard to make, and is mostly irrelevant.
279
280For example, utf8 can be used for code points that don't exist in Unicode, like
2819999999, but if you encode that to UTF-8, you get a substitution character (by
282default; see L<Encode/"Handling Malformed Data"> for more ways of dealing with
283this.)
284
285Okay, if you insist: the "internal format" is utf8, not UTF-8. (When it's not
286some other encoding.)
287
288=head2 I lost track; what encoding is the internal format really?
289
290It's good that you lost track, because you shouldn't depend on the internal
291format being any specific encoding. But since you asked: by default, the
292internal format is either ISO-8859-1 (latin-1), or utf8, depending on the
293history of the string. On EBCDIC platforms, this may be different even.
294
295Perl knows how it stored the string internally, and will use that knowledge
296when you C<encode>. In other words: don't try to find out what the internal
297encoding for a certain string is, but instead just encode it into the encoding
298that you want.
299
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300=head1 AUTHOR
301
740d4bb2 302Juerd Waalboer <#####@juerd.nl>
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303
304=head1 SEE ALSO
305
306L<perlunicode>, L<perluniintro>, L<Encode>
307