1PERLUNIFAQ(1)          Perl Programmers Reference Guide          PERLUNIFAQ(1)
2
3
4

NAME

6       perlunifaq - Perl Unicode FAQ
7

Q and A

9       This is a list of questions and answers about Unicode in Perl, intended
10       to be read after perlunitut.
11
12   perlunitut isn't really a Unicode tutorial, is it?
13       No, and this isn't really a Unicode FAQ.
14
15       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
21   What character encodings does Perl support?
22       To find out which character encodings your Perl supports, run:
23
24           perl -MEncode -le "print for Encode->encodings(':all')"
25
26   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
30       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
34   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
39       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
44   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
50   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
59       This silent implicit decoding is known as "upgrading". That may sound
60       positive, but it's best to avoid it.
61
62   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
67           Wide character in print at example.pl line 2.
68
69       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
75   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
82       You can provide this layer when "open"ing the file:
83
84         open my $fh, '>:encoding(UTF-8)', $filename;  # auto encoding on write
85         open my $fh, '<:encoding(UTF-8)', $filename;  # auto decoding on read
86
87       Or if you already have an open filehandle:
88
89         binmode $fh, ':encoding(UTF-8)';
90
91       Some database drivers for DBI can also automatically encode and decode,
92       but that is sometimes limited to the UTF-8 encoding.
93
94   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
98       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
102       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
106   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
110           use utf8;
111
112       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
117   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
123       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
129       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
132   Why do regex character classes sometimes match only in the ASCII range?
133       Starting in Perl 5.14 (and partially in Perl 5.12), just put a "use
134       feature 'unicode_strings'" near the beginning of your program.  Within
135       its lexical scope you shouldn't have this problem.  It also is
136       automatically enabled under "use feature ':5.12'" or "use v5.12" or
137       using "-E" on the command line for Perl 5.12 or higher.
138
139       The rationale for requiring this is to not break older programs that
140       rely on the way things worked before Unicode came along.  Those older
141       programs knew only about the ASCII character set, and so may not work
142       properly for additional characters.  When a string is encoded in UTF-8,
143       Perl assumes that the program is prepared to deal with Unicode, but
144       when the string isn't, Perl assumes that only ASCII is wanted, and so
145       those characters that are not ASCII characters aren't recognized as to
146       what they would be in Unicode.  "use feature 'unicode_strings'" tells
147       Perl to treat all characters as Unicode, whether the string is encoded
148       in UTF-8 or not, thus avoiding the problem.
149
150       However, on earlier Perls, or if you pass strings to subroutines
151       outside the feature's scope, you can force Unicode rules by changing
152       the encoding to UTF-8 by doing "utf8::upgrade($string)". This can be
153       used safely on any string, as it checks and does not change strings
154       that have already been upgraded.
155
156       For a more detailed discussion, see Unicode::Semantics on CPAN.
157
158   Why do some characters not uppercase or lowercase correctly?
159       See the answer to the previous question.
160
161   How can I determine if a string is a text string or a binary string?
162       You can't. Some use the UTF8 flag for this, but that's misuse, and
163       makes well behaved modules like Data::Dumper look bad. The flag is
164       useless for this purpose, because it's off when an 8 bit encoding (by
165       default ISO-8859-1) is used to store the string.
166
167       This is something you, the programmer, has to keep track of; sorry. You
168       could consider adopting a kind of "Hungarian notation" to help with
169       this.
170
171   How do I convert from encoding FOO to encoding BAR?
172       By first converting the FOO-encoded byte string to a text string, and
173       then the text string to a BAR-encoded byte string:
174
175           my $text_string = decode('FOO', $foo_string);
176           my $bar_string  = encode('BAR', $text_string);
177
178       or by skipping the text string part, and going directly from one binary
179       encoding to the other:
180
181           use Encode qw(from_to);
182           from_to($string, 'FOO', 'BAR');  # changes contents of $string
183
184       or by letting automatic decoding and encoding do all the work:
185
186           open my $foofh, '<:encoding(FOO)', 'example.foo.txt';
187           open my $barfh, '>:encoding(BAR)', 'example.bar.txt';
188           print { $barfh } $_ while <$foofh>;
189
190   What are "decode_utf8" and "encode_utf8"?
191       These are alternate syntaxes for "decode('utf8', ...)" and
192       "encode('utf8', ...)". Do not use these functions for data exchange.
193       Instead use "decode('UTF-8', ...)" and "encode('UTF-8', ...)"; see
194       "What's the difference between UTF-8 and utf8?" below.
195
196   What is a "wide character"?
197       This is a term used for characters occupying more than one byte.
198
199       The Perl warning "Wide character in ..." is caused by such a character.
200       With no specified encoding layer, Perl tries to fit things into a
201       single byte.  When it can't, it emits this warning (if warnings are
202       enabled), and uses UTF-8 encoded data instead.
203
204       To avoid this warning and to avoid having different output encodings in
205       a single stream, always specify an encoding explicitly, for example
206       with a PerlIO layer:
207
208           binmode STDOUT, ":encoding(UTF-8)";
209

INTERNALS

211   What is "the UTF8 flag"?
212       Please, unless you're hacking the internals, or debugging weirdness,
213       don't think about the UTF8 flag at all. That means that you very
214       probably shouldn't use "is_utf8", "_utf8_on" or "_utf8_off" at all.
215
216       The UTF8 flag, also called SvUTF8, is an internal flag that indicates
217       that the current internal representation is UTF-8. Without the flag, it
218       is assumed to be ISO-8859-1. Perl converts between these automatically.
219       (Actually Perl usually assumes the representation is ASCII; see "Why do
220       regex character classes sometimes match only in the ASCII range?"
221       above.)
222
223       One of Perl's internal formats happens to be UTF-8. Unfortunately, Perl
224       can't keep a secret, so everyone knows about this. That is the source
225       of much confusion. It's better to pretend that the internal format is
226       some unknown encoding, and that you always have to encode and decode
227       explicitly.
228
229   What about the "use bytes" pragma?
230       Don't use it. It makes no sense to deal with bytes in a text string,
231       and it makes no sense to deal with characters in a byte string. Do the
232       proper conversions (by decoding/encoding), and things will work out
233       well: you get character counts for decoded data, and byte counts for
234       encoded data.
235
236       "use bytes" is usually a failed attempt to do something useful. Just
237       forget about it.
238
239   What about the "use encoding" pragma?
240       Don't use it. Unfortunately, it assumes that the programmer's
241       environment and that of the user will use the same encoding. It will
242       use the same encoding for the source code and for STDIN and STDOUT.
243       When a program is copied to another machine, the source code does not
244       change, but the STDIO environment might.
245
246       If you need non-ASCII characters in your source code, make it a UTF-8
247       encoded file and "use utf8".
248
249       If you need to set the encoding for STDIN, STDOUT, and STDERR, for
250       example based on the user's locale, "use open".
251
252   What is the difference between ":encoding" and ":utf8"?
253       Because UTF-8 is one of Perl's internal formats, you can often just
254       skip the encoding or decoding step, and manipulate the UTF8 flag
255       directly.
256
257       Instead of ":encoding(UTF-8)", you can simply use ":utf8", which skips
258       the encoding step if the data was already represented as UTF8
259       internally. This is widely accepted as good behavior when you're
260       writing, but it can be dangerous when reading, because it causes
261       internal inconsistency when you have invalid byte sequences. Using
262       ":utf8" for input can sometimes result in security breaches, so please
263       use ":encoding(UTF-8)" instead.
264
265       Instead of "decode" and "encode", you could use "_utf8_on" and
266       "_utf8_off", but this is considered bad style. Especially "_utf8_on"
267       can be dangerous, for the same reason that ":utf8" can.
268
269       There are some shortcuts for oneliners; see -C in perlrun.
270
271   What's the difference between "UTF-8" and "utf8"?
272       "UTF-8" is the official standard. "utf8" is Perl's way of being liberal
273       in what it accepts. If you have to communicate with things that aren't
274       so liberal, you may want to consider using "UTF-8". If you have to
275       communicate with things that are too liberal, you may have to use
276       "utf8". The full explanation is in "UTF-8 vs. utf8 vs. UTF8" in Encode.
277
278       "UTF-8" is internally known as "utf-8-strict". The tutorial uses UTF-8
279       consistently, even where utf8 is actually used internally, because the
280       distinction can be hard to make, and is mostly irrelevant.
281
282       For example, utf8 can be used for code points that don't exist in
283       Unicode, like 9999999, but if you encode that to UTF-8, you get a
284       substitution character (by default; see "Handling Malformed Data" in
285       Encode for more ways of dealing with this.)
286
287       Okay, if you insist: the "internal format" is utf8, not UTF-8. (When
288       it's not some other encoding.)
289
290   I lost track; what encoding is the internal format really?
291       It's good that you lost track, because you shouldn't depend on the
292       internal format being any specific encoding. But since you asked: by
293       default, the internal format is either ISO-8859-1 (latin-1), or utf8,
294       depending on the history of the string. On EBCDIC platforms, this may
295       be different even.
296
297       Perl knows how it stored the string internally, and will use that
298       knowledge when you "encode". In other words: don't try to find out what
299       the internal encoding for a certain string is, but instead just encode
300       it into the encoding that you want.
301

AUTHOR

303       Juerd Waalboer <#####@juerd.nl>
304

SEE ALSO

306       perlunicode, perluniintro, Encode
307
308
309
310perl v5.26.3                      2018-03-23                     PERLUNIFAQ(1)
Impressum