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 they
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 are based on the status quo as of C<5.8.8>.
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 It seemed like a good idea at the time, to keep the semantics the same for
142 standard strings, when Perl got Unicode support. While it might be repaired
143 in the future, we now have to deal with the fact that Perl treats equal
144 strings differently, depending on the internal state.
146 Affected are C<uc>, C<lc>, C<ucfirst>, C<lcfirst>, C<\U>, C<\L>, C<\u>, C<\l>,
147 C<\d>, C<\s>, C<\w>, C<\D>, C<\S>, C<\W>, C</.../i>, C<(?i:...)>,
150 To force Unicode semantics, you can upgrade the internal representation to
151 by doing C<utf8::upgrade($string)>. This does not change strings that were
154 For a more detailed discussion, see L<Unicode::Semantics> on CPAN.
156 =head2 How can I determine if a string is a text string or a binary string?
158 You can't. Some use the UTF8 flag for this, but that's misuse, and makes well
159 behaved modules like Data::Dumper look bad. The flag is useless for this
160 purpose, because it's off when an 8 bit encoding (by default ISO-8859-1) is
161 used to store the string.
163 This is something you, the programmer, has to keep track of; sorry. You could
164 consider adopting a kind of "Hungarian notation" to help with this.
166 =head2 How do I convert from encoding FOO to encoding BAR?
168 By first converting the FOO-encoded byte string to a text string, and then the
169 text string to a BAR-encoded byte string:
171 my $text_string = decode('FOO', $foo_string);
172 my $bar_string = encode('BAR', $text_string);
174 or by skipping the text string part, and going directly from one binary
175 encoding to the other:
177 use Encode qw(from_to);
178 from_to($string, 'FOO', 'BAR'); # changes contents of $string
180 or by letting automatic decoding and encoding do all the work:
182 open my $foofh, '<:encoding(FOO)', 'example.foo.txt';
183 open my $barfh, '>:encoding(BAR)', 'example.bar.txt';
184 print { $barfh } $_ while <$foofh>;
186 =head2 What are C<decode_utf8> and C<encode_utf8>?
188 These are alternate syntaxes for C<decode('utf8', ...)> and C<encode('utf8',
191 =head2 What is a "wide character"?
193 This is a term used both for characters with an ordinal value greater than 127,
194 characters with an ordinal value greater than 255, or any character occupying
195 than one byte, depending on the context.
197 The Perl warning "Wide character in ..." is caused by a character with an
198 ordinal value greater than 255. With no specified encoding layer, Perl tries to
199 fit things in ISO-8859-1 for backward compatibility reasons. When it can't, it
200 emits this warning (if warnings are enabled), and outputs UTF-8 encoded data
203 To avoid this warning and to avoid having different output encodings in a single
204 stream, always specify an encoding explicitly, for example with a PerlIO layer:
206 binmode STDOUT, ":encoding(UTF-8)";
210 =head2 What is "the UTF8 flag"?
212 Please, unless you're hacking the internals, or debugging weirdness, don't
213 think about the UTF8 flag at all. That means that you very probably shouldn't
214 use C<is_utf8>, C<_utf8_on> or C<_utf8_off> at all.
216 The UTF8 flag, also called SvUTF8, is an internal flag that indicates that the
217 current internal representation is UTF-8. Without the flag, it is assumed to be
218 ISO-8859-1. Perl converts between these automatically.
220 One of Perl's internal formats happens to be UTF-8. Unfortunately, Perl can't
221 keep a secret, so everyone knows about this. That is the source of much
222 confusion. It's better to pretend that the internal format is some unknown
223 encoding, and that you always have to encode and decode explicitly.
225 =head2 What about the C<use bytes> pragma?
227 Don't use it. It makes no sense to deal with bytes in a text string, and it
228 makes no sense to deal with characters in a byte string. Do the proper
229 conversions (by decoding/encoding), and things will work out well: you get
230 character counts for decoded data, and byte counts for encoded data.
232 C<use bytes> is usually a failed attempt to do something useful. Just forget
235 =head2 What about the C<use encoding> pragma?
237 Don't use it. Unfortunately, it assumes that the programmer's environment and
238 that of the user will use the same encoding. It will use the same encoding for
239 the source code and for STDIN and STDOUT. When a program is copied to another
240 machine, the source code does not change, but the STDIO environment might.
242 If you need non-ASCII characters in your source code, make it a UTF-8 encoded
243 file and C<use utf8>.
245 If you need to set the encoding for STDIN, STDOUT, and STDERR, for example
246 based on the user's locale, C<use open>.
248 =head2 What is the difference between C<:encoding> and C<:utf8>?
250 Because UTF-8 is one of Perl's internal formats, you can often just skip the
251 encoding or decoding step, and manipulate the UTF8 flag directly.
253 Instead of C<:encoding(UTF-8)>, you can simply use C<:utf8>, which skips the
254 encoding step if the data was already represented as UTF8 internally. This is
255 widely accepted as good behavior when you're writing, but it can be dangerous
256 when reading, because it causes internal inconsistency when you have invalid
257 byte sequences. Using C<:utf8> for input can sometimes result in security
258 breaches, so please use C<:encoding(UTF-8)> instead.
260 Instead of C<decode> and C<encode>, you could use C<_utf8_on> and C<_utf8_off>,
261 but this is considered bad style. Especially C<_utf8_on> can be dangerous, for
262 the same reason that C<:utf8> can.
264 There are some shortcuts for oneliners; see C<-C> in L<perlrun>.
266 =head2 What's the difference between C<UTF-8> and C<utf8>?
268 C<UTF-8> is the official standard. C<utf8> is Perl's way of being liberal in
269 what it accepts. If you have to communicate with things that aren't so liberal,
270 you may want to consider using C<UTF-8>. If you have to communicate with things
271 that are too liberal, you may have to use C<utf8>. The full explanation is in
274 C<UTF-8> is internally known as C<utf-8-strict>. The tutorial uses UTF-8
275 consistently, even where utf8 is actually used internally, because the
276 distinction can be hard to make, and is mostly irrelevant.
278 For example, utf8 can be used for code points that don't exist in Unicode, like
279 9999999, but if you encode that to UTF-8, you get a substitution character (by
280 default; see L<Encode/"Handling Malformed Data"> for more ways of dealing with
283 Okay, if you insist: the "internal format" is utf8, not UTF-8. (When it's not
284 some other encoding.)
286 =head2 I lost track; what encoding is the internal format really?
288 It's good that you lost track, because you shouldn't depend on the internal
289 format being any specific encoding. But since you asked: by default, the
290 internal format is either ISO-8859-1 (latin-1), or utf8, depending on the
291 history of the string. On EBCDIC platforms, this may be different even.
293 Perl knows how it stored the string internally, and will use that knowledge
294 when you C<encode>. In other words: don't try to find out what the internal
295 encoding for a certain string is, but instead just encode it into the encoding
300 Juerd Waalboer <#####@juerd.nl>
304 L<perlunicode>, L<perluniintro>, L<Encode>