1PERLUNIFAQ(1) Perl Programmers Reference Guide PERLUNIFAQ(1)
2
6 perlunifaq - Perl Unicode FAQ
7
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 It depends on what you output and how you output it.
64 Output via a filehandle
66 • If the string's characters are all code point 255 or lower, Perl
68 outputs bytes that match those code points. This is what happens
69 with encoded strings. It can also, though, happen with unencoded
70 strings that happen to be all code point 255 or lower.
71 • Otherwise, Perl outputs the string encoded as UTF-8. This only
73 happens with strings you neglected to encode. Since that should not
74 happen, Perl also throws a "wide character" warning in this case.
75 Other output mechanisms (e.g., "exec", "chdir", ..)
77 Your text string will be sent using the bytes in Perl's internal
79 format.
80 Because the internal format is often UTF-8, these bugs are hard to
82 spot, because UTF-8 is usually the encoding you wanted! But don't be
83 lazy, and don't use the fact that Perl's internal format is UTF-8 to
84 your advantage. Encode explicitly to avoid weird bugs, and to show to
85 maintenance programmers that you thought this through.
86 Is there a way to automatically decode or encode?
88 If all data that comes from a certain handle is encoded in exactly the
89 same way, you can tell the PerlIO system to automatically decode
90 everything, with the "encoding" layer. If you do this, you can't
91 accidentally forget to decode or encode anymore, on things that use the
92 layered handle.
93 You can provide this layer when "open"ing the file:
95 open my $fh, '>:encoding(UTF-8)', $filename; # auto encoding on write
97 open my $fh, '<:encoding(UTF-8)', $filename; # auto decoding on read
98 Or if you already have an open filehandle:
100 binmode $fh, ':encoding(UTF-8)';
102 Some database drivers for DBI can also automatically encode and decode,
104 but that is sometimes limited to the UTF-8 encoding.
105 What if I don't know which encoding was used?
107 Do whatever you can to find out, and if you have to: guess. (Don't
108 forget to document your guess with a comment.)
109 You could open the document in a web browser, and change the character
111 set or character encoding until you can visually confirm that all
112 characters look the way they should.
113 There is no way to reliably detect the encoding automatically, so if
115 people keep sending you data without charset indication, you may have
116 to educate them.
117 Can I use Unicode in my Perl sources?
119 Yes, you can! If your sources are UTF-8 encoded, you can indicate that
120 with the "use utf8" pragma.
121 use utf8;
123 This doesn't do anything to your input, or to your output. It only
125 influences the way your sources are read. You can use Unicode in string
126 literals, in identifiers (but they still have to be "word characters"
127 according to "\w"), and even in custom delimiters.
128 Data::Dumper doesn't restore the UTF8 flag; is it broken?
130 No, Data::Dumper's Unicode abilities are as they should be. There have
131 been some complaints that it should restore the UTF8 flag when the data
132 is read again with "eval". However, you should really not look at the
133 flag, and nothing indicates that Data::Dumper should break this rule.
134 Here's what happens: when Perl reads in a string literal, it sticks to
136 8 bit encoding as long as it can. (But perhaps originally it was
137 internally encoded as UTF-8, when you dumped it.) When it has to give
138 that up because other characters are added to the text string, it
139 silently upgrades the string to UTF-8.
140 If you properly encode your strings for output, none of this is of your
142 concern, and you can just "eval" dumped data as always.
143 Why do regex character classes sometimes match only in the ASCII range?
145 Starting in Perl 5.14 (and partially in Perl 5.12), just put a "use
146 feature 'unicode_strings'" near the beginning of your program. Within
147 its lexical scope you shouldn't have this problem. It also is
148 automatically enabled under "use feature ':5.12'" or "use v5.12" or
149 using "-E" on the command line for Perl 5.12 or higher.
150 The rationale for requiring this is to not break older programs that
152 rely on the way things worked before Unicode came along. Those older
153 programs knew only about the ASCII character set, and so may not work
154 properly for additional characters. When a string is encoded in UTF-8,
155 Perl assumes that the program is prepared to deal with Unicode, but
156 when the string isn't, Perl assumes that only ASCII is wanted, and so
157 those characters that are not ASCII characters aren't recognized as to
158 what they would be in Unicode. "use feature 'unicode_strings'" tells
159 Perl to treat all characters as Unicode, whether the string is encoded
160 in UTF-8 or not, thus avoiding the problem.
161 However, on earlier Perls, or if you pass strings to subroutines
163 outside the feature's scope, you can force Unicode rules by changing
164 the encoding to UTF-8 by doing "utf8::upgrade($string)". This can be
165 used safely on any string, as it checks and does not change strings
166 that have already been upgraded.
167 For a more detailed discussion, see Unicode::Semantics on CPAN.
169 Why do some characters not uppercase or lowercase correctly?
171 See the answer to the previous question.
172 How can I determine if a string is a text string or a binary string?
174 You can't. Some use the UTF8 flag for this, but that's misuse, and
175 makes well behaved modules like Data::Dumper look bad. The flag is
176 useless for this purpose, because it's off when an 8 bit encoding (by
177 default ISO-8859-1) is used to store the string.
178 This is something you, the programmer, has to keep track of; sorry. You
180 could consider adopting a kind of "Hungarian notation" to help with
181 this.
182 How do I convert from encoding FOO to encoding BAR?
184 By first converting the FOO-encoded byte string to a text string, and
185 then the text string to a BAR-encoded byte string:
186 my $text_string = decode('FOO', $foo_string);
188 my $bar_string = encode('BAR', $text_string);
189 or by skipping the text string part, and going directly from one binary
191 encoding to the other:
192 use Encode qw(from_to);
194 from_to($string, 'FOO', 'BAR'); # changes contents of $string
195 or by letting automatic decoding and encoding do all the work:
197 open my $foofh, '<:encoding(FOO)', 'example.foo.txt';
199 open my $barfh, '>:encoding(BAR)', 'example.bar.txt';
200 print { $barfh } $_ while <$foofh>;
201 What are "decode_utf8" and "encode_utf8"?
203 These are alternate syntaxes for "decode('utf8', ...)" and
204 "encode('utf8', ...)". Do not use these functions for data exchange.
205 Instead use "decode('UTF-8', ...)" and "encode('UTF-8', ...)"; see
206 "What's the difference between UTF-8 and utf8?" below.
207 What is a "wide character"?
209 This is a term used for characters occupying more than one byte.
210 The Perl warning "Wide character in ..." is caused by such a character.
212 With no specified encoding layer, Perl tries to fit things into a
213 single byte. When it can't, it emits this warning (if warnings are
214 enabled), and uses UTF-8 encoded data instead.
215 To avoid this warning and to avoid having different output encodings in
217 a single stream, always specify an encoding explicitly, for example
218 with a PerlIO layer:
219 binmode STDOUT, ":encoding(UTF-8)";
221
223 What is "the UTF8 flag"?
224 Please, unless you're hacking the internals, or debugging weirdness,
225 don't think about the UTF8 flag at all. That means that you very
226 probably shouldn't use "is_utf8", "_utf8_on" or "_utf8_off" at all.
227 The UTF8 flag, also called SvUTF8, is an internal flag that indicates
229 that the current internal representation is UTF-8. Without the flag, it
230 is assumed to be ISO-8859-1. Perl converts between these automatically.
231 (Actually Perl usually assumes the representation is ASCII; see "Why do
232 regex character classes sometimes match only in the ASCII range?"
233 above.)
234 One of Perl's internal formats happens to be UTF-8. Unfortunately, Perl
236 can't keep a secret, so everyone knows about this. That is the source
237 of much confusion. It's better to pretend that the internal format is
238 some unknown encoding, and that you always have to encode and decode
239 explicitly.
240 What about the "use bytes" pragma?
242 Don't use it. It makes no sense to deal with bytes in a text string,
243 and it makes no sense to deal with characters in a byte string. Do the
244 proper conversions (by decoding/encoding), and things will work out
245 well: you get character counts for decoded data, and byte counts for
246 encoded data.
247 "use bytes" is usually a failed attempt to do something useful. Just
249 forget about it.
250 What about the "use encoding" pragma?
252 Don't use it. Unfortunately, it assumes that the programmer's
253 environment and that of the user will use the same encoding. It will
254 use the same encoding for the source code and for STDIN and STDOUT.
255 When a program is copied to another machine, the source code does not
256 change, but the STDIO environment might.
257 If you need non-ASCII characters in your source code, make it a UTF-8
259 encoded file and "use utf8".
260 If you need to set the encoding for STDIN, STDOUT, and STDERR, for
262 example based on the user's locale, "use open".
263 What is the difference between ":encoding" and ":utf8"?
265 Because UTF-8 is one of Perl's internal formats, you can often just
266 skip the encoding or decoding step, and manipulate the UTF8 flag
267 directly.
268 Instead of ":encoding(UTF-8)", you can simply use ":utf8", which skips
270 the encoding step if the data was already represented as UTF8
271 internally. This is widely accepted as good behavior when you're
272 writing, but it can be dangerous when reading, because it causes
273 internal inconsistency when you have invalid byte sequences. Using
274 ":utf8" for input can sometimes result in security breaches, so please
275 use ":encoding(UTF-8)" instead.
276 Instead of "decode" and "encode", you could use "_utf8_on" and
278 "_utf8_off", but this is considered bad style. Especially "_utf8_on"
279 can be dangerous, for the same reason that ":utf8" can.
280 There are some shortcuts for oneliners; see -C in perlrun.
282 What's the difference between "UTF-8" and "utf8"?
284 "UTF-8" is the official standard. "utf8" is Perl's way of being liberal
285 in what it accepts. If you have to communicate with things that aren't
286 so liberal, you may want to consider using "UTF-8". If you have to
287 communicate with things that are too liberal, you may have to use
288 "utf8". The full explanation is in "UTF-8 vs. utf8 vs. UTF8" in Encode.
289 "UTF-8" is internally known as "utf-8-strict". The tutorial uses UTF-8
291 consistently, even where utf8 is actually used internally, because the
292 distinction can be hard to make, and is mostly irrelevant.
293 For example, utf8 can be used for code points that don't exist in
295 Unicode, like 9999999, but if you encode that to UTF-8, you get a
296 substitution character (by default; see "Handling Malformed Data" in
297 Encode for more ways of dealing with this.)
298 Okay, if you insist: the "internal format" is utf8, not UTF-8. (When
300 it's not some other encoding.)
301 I lost track; what encoding is the internal format really?
303 It's good that you lost track, because you shouldn't depend on the
304 internal format being any specific encoding. But since you asked: by
305 default, the internal format is either ISO-8859-1 (latin-1), or utf8,
306 depending on the history of the string. On EBCDIC platforms, this may
307 be different even.
308 Perl knows how it stored the string internally, and will use that
310 knowledge when you "encode". In other words: don't try to find out what
311 the internal encoding for a certain string is, but instead just encode
312 it into the encoding that you want.
313
315 Juerd Waalboer <#####@juerd.nl>
316
318 perlunicode, perluniintro, Encode
319perl v5.34.1 2022-03-15 PERLUNIFAQ(1)