3 perlunifaq - Perl Unicode FAQ
7 This is a list of questions and answers about Unicode in Perl, intended to be
8 read after L<perlunitut>.
10 =head2 perlunitut isn't really a Unicode tutorial, is it?
12 No, and this isn't really a Unicode FAQ.
14 Perl has an abstracted interface for all supported character encodings, so this
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.
19 =head2 What character encodings does Perl support?
21 To find out which character encodings your Perl supports, run:
23 perl -MEncode -le "print for Encode->encodings(':all')"
25 =head2 Which version of perl should I use?
27 Well, if you can, upgrade to the most recent, but certainly C<5.8.1> or newer.
28 The tutorial and FAQ assume the latest release.
30 You should also check your modules, and upgrade them if necessary. For example,
31 HTML::Entities requires version >= 1.32 to function correctly, even though the
32 changelog is silent about this.
34 =head2 What about binary data, like images?
36 Well, 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
40 Be careful, though, to never combine text strings with binary strings. If you
41 need text in a binary stream, encode your text strings first using the
42 appropriate encoding, then join them with binary strings. See also: "What if I
45 =head2 When should I decode or encode?
47 Whenever you're communicating text with anything that is external to your perl
48 process, like a database, a text file, a socket, or another program. Even if
49 the thing you're communicating with is also written in Perl.
51 =head2 What if I don't decode?
53 Whenever your encoded, binary string is used together with a text string, Perl
54 will assume that your binary string was encoded with ISO-8859-1, also known as
55 latin-1. If it wasn't latin-1, then your data is unpleasantly converted. For
56 example, if it was UTF-8, the individual bytes of multibyte characters are seen
57 as separate characters, and then again converted to UTF-8. Such double encoding
58 can be compared to double HTML encoding (C<&gt;>), or double URI encoding
61 This silent implicit decoding is known as "upgrading". That may sound
62 positive, but it's best to avoid it.
64 =head2 What if I don't encode?
66 Your text string will be sent using the bytes in Perl's internal format. In
67 some cases, Perl will warn you that you're doing something wrong, with a
70 Wide character in print at example.pl line 2.
72 Because the internal format is often UTF-8, these bugs are hard to spot,
73 because UTF-8 is usually the encoding you wanted! But don't be lazy, and don't
74 use the fact that Perl's internal format is UTF-8 to your advantage. Encode
75 explicitly to avoid weird bugs, and to show to maintenance programmers that you
78 =head2 Is there a way to automatically decode or encode?
80 If all data that comes from a certain handle is encoded in exactly the same
81 way, you can tell the PerlIO system to automatically decode everything, with
82 the C<encoding> layer. If you do this, you can't accidentally forget to decode
83 or encode anymore, on things that use the layered handle.
85 You can provide this layer when C<open>ing the file:
87 open my $fh, '>:encoding(UTF-8)', $filename; # auto encoding on write
88 open my $fh, '<:encoding(UTF-8)', $filename; # auto decoding on read
90 Or if you already have an open filehandle:
92 binmode $fh, ':encoding(UTF-8)';
94 Some database drivers for DBI can also automatically encode and decode, but
95 that is sometimes limited to the UTF-8 encoding.
97 =head2 What if I don't know which encoding was used?
99 Do whatever you can to find out, and if you have to: guess. (Don't forget to
100 document your guess with a comment.)
102 You could open the document in a web browser, and change the character set or
103 character encoding until you can visually confirm that all characters look the
106 There is no way to reliably detect the encoding automatically, so if people
107 keep sending you data without charset indication, you may have to educate them.
109 =head2 Can I use Unicode in my Perl sources?
111 Yes, you can! If your sources are UTF-8 encoded, you can indicate that with the
116 This doesn't do anything to your input, or to your output. It only influences
117 the way your sources are read. You can use Unicode in string literals, in
118 identifiers (but they still have to be "word characters" according to C<\w>),
119 and even in custom delimiters.
121 =head2 Data::Dumper doesn't restore the UTF8 flag; is it broken?
123 No, Data::Dumper's Unicode abilities are as they should be. There have been
124 some complaints that it should restore the UTF8 flag when the data is read
125 again with C<eval>. However, you should really not look at the flag, and
126 nothing indicates that Data::Dumper should break this rule.
128 Here's what happens: when Perl reads in a string literal, it sticks to 8 bit
129 encoding as long as it can. (But perhaps originally it was internally encoded
130 as UTF-8, when you dumped it.) When it has to give that up because other
131 characters are added to the text string, it silently upgrades the string to
134 If you properly encode your strings for output, none of this is of your
135 concern, and you can just C<eval> dumped data as always.
137 =head2 Why do regex character classes sometimes match only in the ASCII range?
139 =head2 Why do some characters not uppercase or lowercase correctly?
141 Starting in Perl 5.14 (and partially in Perl 5.12), just put a
142 C<use feature 'unicode_strings'> near the beginning of your program.
143 Within its lexical scope you shouldn't have this problem. It also is
144 automatically enabled under C<use feature ':5.12'> or using C<-E> on the
145 command line for Perl 5.12 or higher.
147 The rationale for requiring this is to not break older programs that
148 rely on the way things worked before Unicode came along. Those older
149 programs knew only about the ASCII character set, and so may not work
150 properly for additional characters. When a string is encoded in UTF-8,
151 Perl assumes that the program is prepared to deal with Unicode, but when
152 the string isn't, Perl assumes that only ASCII (unless it is an EBCDIC
153 platform) is wanted, and so those characters that are not ASCII
154 characters aren't recognized as to what they would be in Unicode.
155 C<use feature 'unicode_strings'> tells Perl to treat all characters as
156 Unicode, whether the string is encoded in UTF-8 or not, thus avoiding
159 However, on earlier Perls, or if you pass strings to subroutines outside
160 the feature's scope, you can force Unicode semantics by changing the
161 encoding to UTF-8 by doing C<utf8::upgrade($string)>. This can be used
162 safely on any string, as it checks and does not change strings that have
163 already been upgraded.
165 For a more detailed discussion, see L<Unicode::Semantics> on CPAN.
167 =head2 How can I determine if a string is a text string or a binary string?
169 You can't. Some use the UTF8 flag for this, but that's misuse, and makes well
170 behaved modules like Data::Dumper look bad. The flag is useless for this
171 purpose, because it's off when an 8 bit encoding (by default ISO-8859-1) is
172 used to store the string.
174 This is something you, the programmer, has to keep track of; sorry. You could
175 consider adopting a kind of "Hungarian notation" to help with this.
177 =head2 How do I convert from encoding FOO to encoding BAR?
179 By first converting the FOO-encoded byte string to a text string, and then the
180 text string to a BAR-encoded byte string:
182 my $text_string = decode('FOO', $foo_string);
183 my $bar_string = encode('BAR', $text_string);
185 or by skipping the text string part, and going directly from one binary
186 encoding to the other:
188 use Encode qw(from_to);
189 from_to($string, 'FOO', 'BAR'); # changes contents of $string
191 or by letting automatic decoding and encoding do all the work:
193 open my $foofh, '<:encoding(FOO)', 'example.foo.txt';
194 open my $barfh, '>:encoding(BAR)', 'example.bar.txt';
195 print { $barfh } $_ while <$foofh>;
197 =head2 What are C<decode_utf8> and C<encode_utf8>?
199 These are alternate syntaxes for C<decode('utf8', ...)> and C<encode('utf8',
202 =head2 What is a "wide character"?
204 This is a term used both for characters with an ordinal value greater than 127,
205 characters with an ordinal value greater than 255, or any character occupying
206 more than one byte, depending on the context.
208 The Perl warning "Wide character in ..." is caused by a character with an
209 ordinal value greater than 255. With no specified encoding layer, Perl tries to
210 fit things in ISO-8859-1 for backward compatibility reasons. When it can't, it
211 emits this warning (if warnings are enabled), and outputs UTF-8 encoded data
214 To avoid this warning and to avoid having different output encodings in a single
215 stream, always specify an encoding explicitly, for example with a PerlIO layer:
217 binmode STDOUT, ":encoding(UTF-8)";
221 =head2 What is "the UTF8 flag"?
223 Please, unless you're hacking the internals, or debugging weirdness, don't
224 think about the UTF8 flag at all. That means that you very probably shouldn't
225 use C<is_utf8>, C<_utf8_on> or C<_utf8_off> at all.
227 The UTF8 flag, also called SvUTF8, is an internal flag that indicates that the
228 current internal representation is UTF-8. Without the flag, it is assumed to be
229 ISO-8859-1. Perl converts between these automatically. (Actually Perl usually
230 assumes the representation is ASCII; see L</Why do regex character classes
231 sometimes match only in the ASCII range?> above.)
233 One of Perl's internal formats happens to be UTF-8. Unfortunately, Perl can't
234 keep a secret, so everyone knows about this. That is the source of much
235 confusion. It's better to pretend that the internal format is some unknown
236 encoding, and that you always have to encode and decode explicitly.
238 =head2 What about the C<use bytes> pragma?
240 Don't use it. It makes no sense to deal with bytes in a text string, and it
241 makes no sense to deal with characters in a byte string. Do the proper
242 conversions (by decoding/encoding), and things will work out well: you get
243 character counts for decoded data, and byte counts for encoded data.
245 C<use bytes> is usually a failed attempt to do something useful. Just forget
248 =head2 What about the C<use encoding> pragma?
250 Don't use it. Unfortunately, it assumes that the programmer's environment and
251 that of the user will use the same encoding. It will use the same encoding for
252 the source code and for STDIN and STDOUT. When a program is copied to another
253 machine, the source code does not change, but the STDIO environment might.
255 If you need non-ASCII characters in your source code, make it a UTF-8 encoded
256 file and C<use utf8>.
258 If you need to set the encoding for STDIN, STDOUT, and STDERR, for example
259 based on the user's locale, C<use open>.
261 =head2 What is the difference between C<:encoding> and C<:utf8>?
263 Because UTF-8 is one of Perl's internal formats, you can often just skip the
264 encoding or decoding step, and manipulate the UTF8 flag directly.
266 Instead of C<:encoding(UTF-8)>, you can simply use C<:utf8>, which skips the
267 encoding step if the data was already represented as UTF8 internally. This is
268 widely accepted as good behavior when you're writing, but it can be dangerous
269 when reading, because it causes internal inconsistency when you have invalid
270 byte sequences. Using C<:utf8> for input can sometimes result in security
271 breaches, so please use C<:encoding(UTF-8)> instead.
273 Instead of C<decode> and C<encode>, you could use C<_utf8_on> and C<_utf8_off>,
274 but this is considered bad style. Especially C<_utf8_on> can be dangerous, for
275 the same reason that C<:utf8> can.
277 There are some shortcuts for oneliners;
278 see L<-C|perlrun/-C [numberE<sol>list]> in L<perlrun>.
280 =head2 What's the difference between C<UTF-8> and C<utf8>?
282 C<UTF-8> is the official standard. C<utf8> is Perl's way of being liberal in
283 what it accepts. If you have to communicate with things that aren't so liberal,
284 you may want to consider using C<UTF-8>. If you have to communicate with things
285 that are too liberal, you may have to use C<utf8>. The full explanation is in
288 C<UTF-8> is internally known as C<utf-8-strict>. The tutorial uses UTF-8
289 consistently, even where utf8 is actually used internally, because the
290 distinction can be hard to make, and is mostly irrelevant.
292 For example, utf8 can be used for code points that don't exist in Unicode, like
293 9999999, but if you encode that to UTF-8, you get a substitution character (by
294 default; see L<Encode/"Handling Malformed Data"> for more ways of dealing with
297 Okay, if you insist: the "internal format" is utf8, not UTF-8. (When it's not
298 some other encoding.)
300 =head2 I lost track; what encoding is the internal format really?
302 It's good that you lost track, because you shouldn't depend on the internal
303 format being any specific encoding. But since you asked: by default, the
304 internal format is either ISO-8859-1 (latin-1), or utf8, depending on the
305 history of the string. On EBCDIC platforms, this may be different even.
307 Perl knows how it stored the string internally, and will use that knowledge
308 when you C<encode>. In other words: don't try to find out what the internal
309 encoding for a certain string is, but instead just encode it into the encoding
314 Juerd Waalboer <#####@juerd.nl>
318 L<perlunicode>, L<perluniintro>, L<Encode>