1PERLUNIFAQ(1) Perl Programmers Reference Guide PERLUNIFAQ(1)
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6 perlunifaq - Perl Unicode FAQ
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9 This is a list of questions and answers about Unicode in Perl, intended
10 to be read after perlunitut.
11 perlunitut isn't really a Unicode tutorial, is it?
13 No, and this isn't really a Unicode FAQ.
14 Perl has an abstracted interface for all supported character encodings,
16 so this is actually a generic "Encode" tutorial and "Encode" FAQ. But
17 many people think that Unicode is special and magical, and I didn't
18 want to disappoint them, so I decided to call the document a Unicode
19 tutorial.
20 What character encodings does Perl support?
22 To find out which character encodings your Perl supports, run:
23 perl -MEncode -le "print for Encode->encodings(':all')"
25 Which version of perl should I use?
27 Well, if you can, upgrade to the most recent, but certainly 5.8.1 or
28 newer. The tutorial and FAQ assume the latest release.
29 You should also check your modules, and upgrade them if necessary. For
31 example, HTML::Entities requires version >= 1.32 to function correctly,
32 even though the changelog is silent about this.
33 What about binary data, like images?
35 Well, apart from a bare "binmode $fh", you shouldn't treat them
36 specially. (The binmode is needed because otherwise Perl may convert
37 line endings on Win32 systems.)
38 Be careful, though, to never combine text strings with binary strings.
40 If you need text in a binary stream, encode your text strings first
41 using the appropriate encoding, then join them with binary strings. See
42 also: "What if I don't encode?".
43 When should I decode or encode?
45 Whenever you're communicating text with anything that is external to
46 your perl process, like a database, a text file, a socket, or another
47 program. Even if the thing you're communicating with is also written in
48 Perl.
49 What if I don't decode?
51 Whenever your encoded, binary string is used together with a text
52 string, Perl will assume that your binary string was encoded with
53 ISO-8859-1, also known as latin-1. If it wasn't latin-1, then your data
54 is unpleasantly converted. For example, if it was UTF-8, the individual
55 bytes of multibyte characters are seen as separate characters, and then
56 again converted to UTF-8. Such double encoding can be compared to
57 double HTML encoding (">"), or double URI encoding (%253E).
58 This silent implicit decoding is known as "upgrading". That may sound
60 positive, but it's best to avoid it.
61 What if I don't encode?
63 Your text string will be sent using the bytes in Perl's internal
64 format. In some cases, Perl will warn you that you're doing something
65 wrong, with a friendly warning:
66 Wide character in print at example.pl line 2.
68 Because the internal format is often UTF-8, these bugs are hard to
70 spot, because UTF-8 is usually the encoding you wanted! But don't be
71 lazy, and don't use the fact that Perl's internal format is UTF-8 to
72 your advantage. Encode explicitly to avoid weird bugs, and to show to
73 maintenance programmers that you thought this through.
74 Is there a way to automatically decode or encode?
76 If all data that comes from a certain handle is encoded in exactly the
77 same way, you can tell the PerlIO system to automatically decode
78 everything, with the "encoding" layer. If you do this, you can't
79 accidentally forget to decode or encode anymore, on things that use the
80 layered handle.
81 You can provide this layer when "open"ing the file:
83 open my $fh, '>:encoding(UTF-8)', $filename; # auto encoding on write
85 open my $fh, '<:encoding(UTF-8)', $filename; # auto decoding on read
86 Or if you already have an open filehandle:
88 binmode $fh, ':encoding(UTF-8)';
90 Some database drivers for DBI can also automatically encode and decode,
92 but that is sometimes limited to the UTF-8 encoding.
93 What if I don't know which encoding was used?
95 Do whatever you can to find out, and if you have to: guess. (Don't
96 forget to document your guess with a comment.)
97 You could open the document in a web browser, and change the character
99 set or character encoding until you can visually confirm that all
100 characters look the way they should.
101 There is no way to reliably detect the encoding automatically, so if
103 people keep sending you data without charset indication, you may have
104 to educate them.
105 Can I use Unicode in my Perl sources?
107 Yes, you can! If your sources are UTF-8 encoded, you can indicate that
108 with the "use utf8" pragma.
109 use utf8;
111 This doesn't do anything to your input, or to your output. It only
113 influences the way your sources are read. You can use Unicode in string
114 literals, in identifiers (but they still have to be "word characters"
115 according to "\w"), and even in custom delimiters.
116 Data::Dumper doesn't restore the UTF8 flag; is it broken?
118 No, Data::Dumper's Unicode abilities are as they should be. There have
119 been some complaints that it should restore the UTF8 flag when the data
120 is read again with "eval". However, you should really not look at the
121 flag, and nothing indicates that Data::Dumper should break this rule.
122 Here's what happens: when Perl reads in a string literal, it sticks to
124 8 bit encoding as long as it can. (But perhaps originally it was
125 internally encoded as UTF-8, when you dumped it.) When it has to give
126 that up because other characters are added to the text string, it
127 silently upgrades the string to UTF-8.
128 If you properly encode your strings for output, none of this is of your
130 concern, and you can just "eval" dumped data as always.
131 Why do regex character classes sometimes match only in the ASCII range?
133 Why do some characters not uppercase or lowercase correctly?
134 It seemed like a good idea at the time, to keep the semantics the same
135 for standard strings, when Perl got Unicode support. The plan is to
136 fix this in the future, and the casing component has in fact mostly
137 been fixed, but we have to deal with the fact that Perl treats equal
138 strings differently, depending on the internal state.
139 First the casing. Just put a "use feature 'unicode_strings'" near the
141 beginning of your program. Within its lexical scope, "uc", "lc",
142 "ucfirst", "lcfirst", and the regular expression escapes "\U", "\L",
143 "\u", "\l" use Unicode semantics for changing case regardless of
144 whether the UTF8 flag is on or not. However, if you pass strings to
145 subroutines in modules outside the pragma's scope, they currently
146 likely won't behave this way, and you have to try one of the solutions
147 below. There is another exception as well: if you have furnished your
148 own casing functions to override the default, these will not be called
149 unless the UTF8 flag is on)
150 This remains a problem for the regular expression constructs "\d",
152 "\s", "\w", "\D", "\S", "\W", "/.../i", "(?i:...)", and
153 "/[[:posix:]]/".
154 To force Unicode semantics, you can upgrade the internal representation
156 to by doing "utf8::upgrade($string)". This can be used safely on any
157 string, as it checks and does not change strings that have already been
158 upgraded.
159 For a more detailed discussion, see Unicode::Semantics on CPAN.
161 How can I determine if a string is a text string or a binary string?
163 You can't. Some use the UTF8 flag for this, but that's misuse, and
164 makes well behaved modules like Data::Dumper look bad. The flag is
165 useless for this purpose, because it's off when an 8 bit encoding (by
166 default ISO-8859-1) is used to store the string.
167 This is something you, the programmer, has to keep track of; sorry. You
169 could consider adopting a kind of "Hungarian notation" to help with
170 this.
171 How do I convert from encoding FOO to encoding BAR?
173 By first converting the FOO-encoded byte string to a text string, and
174 then the text string to a BAR-encoded byte string:
175 my $text_string = decode('FOO', $foo_string);
177 my $bar_string = encode('BAR', $text_string);
178 or by skipping the text string part, and going directly from one binary
180 encoding to the other:
181 use Encode qw(from_to);
183 from_to($string, 'FOO', 'BAR'); # changes contents of $string
184 or by letting automatic decoding and encoding do all the work:
186 open my $foofh, '<:encoding(FOO)', 'example.foo.txt';
188 open my $barfh, '>:encoding(BAR)', 'example.bar.txt';
189 print { $barfh } $_ while <$foofh>;
190 What are "decode_utf8" and "encode_utf8"?
192 These are alternate syntaxes for "decode('utf8', ...)" and
193 "encode('utf8', ...)".
194 What is a "wide character"?
196 This is a term used both for characters with an ordinal value greater
197 than 127, characters with an ordinal value greater than 255, or any
198 character occupying more than one byte, depending on the context.
199 The Perl warning "Wide character in ..." is caused by a character with
201 an ordinal value greater than 255. With no specified encoding layer,
202 Perl tries to fit things in ISO-8859-1 for backward compatibility
203 reasons. When it can't, it emits this warning (if warnings are
204 enabled), and outputs UTF-8 encoded data instead.
205 To avoid this warning and to avoid having different output encodings in
207 a single stream, always specify an encoding explicitly, for example
208 with a PerlIO layer:
209 binmode STDOUT, ":encoding(UTF-8)";
211
213 What is "the UTF8 flag"?
214 Please, unless you're hacking the internals, or debugging weirdness,
215 don't think about the UTF8 flag at all. That means that you very
216 probably shouldn't use "is_utf8", "_utf8_on" or "_utf8_off" at all.
217 The UTF8 flag, also called SvUTF8, is an internal flag that indicates
219 that the current internal representation is UTF-8. Without the flag, it
220 is assumed to be ISO-8859-1. Perl converts between these automatically.
221 (Actually Perl usually assumes the representation is ASCII; see "Why do
222 regex character classes sometimes match only in the ASCII range?"
223 above.)
224 One of Perl's internal formats happens to be UTF-8. Unfortunately, Perl
226 can't keep a secret, so everyone knows about this. That is the source
227 of much confusion. It's better to pretend that the internal format is
228 some unknown encoding, and that you always have to encode and decode
229 explicitly.
230 What about the "use bytes" pragma?
232 Don't use it. It makes no sense to deal with bytes in a text string,
233 and it makes no sense to deal with characters in a byte string. Do the
234 proper conversions (by decoding/encoding), and things will work out
235 well: you get character counts for decoded data, and byte counts for
236 encoded data.
237 "use bytes" is usually a failed attempt to do something useful. Just
239 forget about it.
240 What about the "use encoding" pragma?
242 Don't use it. Unfortunately, it assumes that the programmer's
243 environment and that of the user will use the same encoding. It will
244 use the same encoding for the source code and for STDIN and STDOUT.
245 When a program is copied to another machine, the source code does not
246 change, but the STDIO environment might.
247 If you need non-ASCII characters in your source code, make it a UTF-8
249 encoded file and "use utf8".
250 If you need to set the encoding for STDIN, STDOUT, and STDERR, for
252 example based on the user's locale, "use open".
253 What is the difference between ":encoding" and ":utf8"?
255 Because UTF-8 is one of Perl's internal formats, you can often just
256 skip the encoding or decoding step, and manipulate the UTF8 flag
257 directly.
258 Instead of ":encoding(UTF-8)", you can simply use ":utf8", which skips
260 the encoding step if the data was already represented as UTF8
261 internally. This is widely accepted as good behavior when you're
262 writing, but it can be dangerous when reading, because it causes
263 internal inconsistency when you have invalid byte sequences. Using
264 ":utf8" for input can sometimes result in security breaches, so please
265 use ":encoding(UTF-8)" instead.
266 Instead of "decode" and "encode", you could use "_utf8_on" and
268 "_utf8_off", but this is considered bad style. Especially "_utf8_on"
269 can be dangerous, for the same reason that ":utf8" can.
270 There are some shortcuts for oneliners; see "-C" in perlrun.
272 What's the difference between "UTF-8" and "utf8"?
274 "UTF-8" is the official standard. "utf8" is Perl's way of being liberal
275 in what it accepts. If you have to communicate with things that aren't
276 so liberal, you may want to consider using "UTF-8". If you have to
277 communicate with things that are too liberal, you may have to use
278 "utf8". The full explanation is in Encode.
279 "UTF-8" is internally known as "utf-8-strict". The tutorial uses UTF-8
281 consistently, even where utf8 is actually used internally, because the
282 distinction can be hard to make, and is mostly irrelevant.
283 For example, utf8 can be used for code points that don't exist in
285 Unicode, like 9999999, but if you encode that to UTF-8, you get a
286 substitution character (by default; see "Handling Malformed Data" in
287 Encode for more ways of dealing with this.)
288 Okay, if you insist: the "internal format" is utf8, not UTF-8. (When
290 it's not some other encoding.)
291 I lost track; what encoding is the internal format really?
293 It's good that you lost track, because you shouldn't depend on the
294 internal format being any specific encoding. But since you asked: by
295 default, the internal format is either ISO-8859-1 (latin-1), or utf8,
296 depending on the history of the string. On EBCDIC platforms, this may
297 be different even.
298 Perl knows how it stored the string internally, and will use that
300 knowledge when you "encode". In other words: don't try to find out what
301 the internal encoding for a certain string is, but instead just encode
302 it into the encoding that you want.
303
305 Juerd Waalboer <#####@juerd.nl>
306
308 perlunicode, perluniintro, Encode
309perl v5.12.4 2011-06-07 PERLUNIFAQ(1)