1=head1 NAME 2 3perlunifaq - Perl Unicode FAQ 4 5=head1 Q and A 6 7This is a list of questions and answers about Unicode in Perl, intended to be 8read after L<perlunitut>. 9 10=head2 perlunitut isn't really a Unicode tutorial, is it? 11 12No, and this isn't really a Unicode FAQ. 13 14Perl has an abstracted interface for all supported character encodings, so this 15is actually a generic C<Encode> tutorial and C<Encode> FAQ. But many people 16think that Unicode is special and magical, and I didn't want to disappoint 17them, so I decided to call the document a Unicode tutorial. 18 19=head2 What character encodings does Perl support? 20 21To find out which character encodings your Perl supports, run: 22 23 perl -MEncode -le "print for Encode->encodings(':all')" 24 25=head2 Which version of perl should I use? 26 27Well, if you can, upgrade to the most recent, but certainly C<5.8.1> or newer. 28The tutorial and FAQ assume the latest release. 29 30You should also check your modules, and upgrade them if necessary. For example, 31HTML::Entities requires version >= 1.32 to function correctly, even though the 32changelog is silent about this. 33 34=head2 What about binary data, like images? 35 36Well, 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 38systems.) 39 40Be careful, though, to never combine text strings with binary strings. If you 41need text in a binary stream, encode your text strings first using the 42appropriate encoding, then join them with binary strings. See also: "What if I 43don't encode?". 44 45=head2 When should I decode or encode? 46 47Whenever you're communicating text with anything that is external to your perl 48process, like a database, a text file, a socket, or another program. Even if 49the thing you're communicating with is also written in Perl. 50 51=head2 What if I don't decode? 52 53Whenever your encoded, binary string is used together with a text string, Perl 54will assume that your binary string was encoded with ISO-8859-1, also known as 55latin-1. If it wasn't latin-1, then your data is unpleasantly converted. For 56example, if it was UTF-8, the individual bytes of multibyte characters are seen 57as separate characters, and then again converted to UTF-8. Such double encoding 58can be compared to double HTML encoding (C<&gt;>), or double URI encoding 59(C<%253E>). 60 61This silent implicit decoding is known as "upgrading". That may sound 62positive, but it's best to avoid it. 63 64=head2 What if I don't encode? 65 66It depends on what you output and how you output it. 67 68=head3 Output via a filehandle 69 70=over 71 72=item * If the string's characters are all code point 255 or lower, Perl 73outputs bytes that match those code points. This is what happens with encoded 74strings. It can also, though, happen with unencoded strings that happen to be 75all code point 255 or lower. 76 77=item * Otherwise, Perl outputs the string encoded as UTF-8. This only happens 78with strings you neglected to encode. Since that should not happen, Perl also 79throws a "wide character" warning in this case. 80 81=back 82 83=head3 Other output mechanisms (e.g., C<exec>, C<chdir>, ..) 84 85Your text string will be sent using the bytes in Perl's internal format. 86 87Because the internal format is often UTF-8, these bugs are hard to spot, 88because UTF-8 is usually the encoding you wanted! But don't be lazy, and don't 89use the fact that Perl's internal format is UTF-8 to your advantage. Encode 90explicitly to avoid weird bugs, and to show to maintenance programmers that you 91thought this through. 92 93=head2 Is there a way to automatically decode or encode? 94 95If all data that comes from a certain handle is encoded in exactly the same 96way, you can tell the PerlIO system to automatically decode everything, with 97the C<encoding> layer. If you do this, you can't accidentally forget to decode 98or encode anymore, on things that use the layered handle. 99 100You can provide this layer when C<open>ing the file: 101 102 open my $fh, '>:encoding(UTF-8)', $filename; # auto encoding on write 103 open my $fh, '<:encoding(UTF-8)', $filename; # auto decoding on read 104 105Or if you already have an open filehandle: 106 107 binmode $fh, ':encoding(UTF-8)'; 108 109Some database drivers for DBI can also automatically encode and decode, but 110that is sometimes limited to the UTF-8 encoding. 111 112=head2 What if I don't know which encoding was used? 113 114Do whatever you can to find out, and if you have to: guess. (Don't forget to 115document your guess with a comment.) 116 117You could open the document in a web browser, and change the character set or 118character encoding until you can visually confirm that all characters look the 119way they should. 120 121There is no way to reliably detect the encoding automatically, so if people 122keep sending you data without charset indication, you may have to educate them. 123 124=head2 Can I use Unicode in my Perl sources? 125 126Yes, you can! If your sources are UTF-8 encoded, you can indicate that with the 127C<use utf8> pragma. 128 129 use utf8; 130 131This doesn't do anything to your input, or to your output. It only influences 132the way your sources are read. You can use Unicode in string literals, in 133identifiers (but they still have to be "word characters" according to C<\w>), 134and even in custom delimiters. 135 136=head2 Data::Dumper doesn't restore the UTF8 flag; is it broken? 137 138No, Data::Dumper's Unicode abilities are as they should be. There have been 139some complaints that it should restore the UTF8 flag when the data is read 140again with C<eval>. However, you should really not look at the flag, and 141nothing indicates that Data::Dumper should break this rule. 142 143Here's what happens: when Perl reads in a string literal, it sticks to 8 bit 144encoding as long as it can. (But perhaps originally it was internally encoded 145as UTF-8, when you dumped it.) When it has to give that up because other 146characters are added to the text string, it silently upgrades the string to 147UTF-8. 148 149If you properly encode your strings for output, none of this is of your 150concern, and you can just C<eval> dumped data as always. 151 152=head2 Why do regex character classes sometimes match only in the ASCII range? 153 154Starting in Perl 5.14 (and partially in Perl 5.12), just put a 155C<use feature 'unicode_strings'> near the beginning of your program. 156Within its lexical scope you shouldn't have this problem. It also is 157automatically enabled under C<use feature ':5.12'> or C<use v5.12> or 158using C<-E> on the command line for Perl 5.12 or higher. 159 160The rationale for requiring this is to not break older programs that 161rely on the way things worked before Unicode came along. Those older 162programs knew only about the ASCII character set, and so may not work 163properly for additional characters. When a string is encoded in UTF-8, 164Perl assumes that the program is prepared to deal with Unicode, but when 165the string isn't, Perl assumes that only ASCII 166is wanted, and so those characters that are not ASCII 167characters aren't recognized as to what they would be in Unicode. 168C<use feature 'unicode_strings'> tells Perl to treat all characters as 169Unicode, whether the string is encoded in UTF-8 or not, thus avoiding 170the problem. 171 172However, on earlier Perls, or if you pass strings to subroutines outside 173the feature's scope, you can force Unicode rules by changing the 174encoding to UTF-8 by doing C<utf8::upgrade($string)>. This can be used 175safely on any string, as it checks and does not change strings that have 176already been upgraded. 177 178For a more detailed discussion, see L<Unicode::Semantics> on CPAN. 179 180=head2 Why do some characters not uppercase or lowercase correctly? 181 182See the answer to the previous question. 183 184=head2 How can I determine if a string is a text string or a binary string? 185 186You can't. Some use the UTF8 flag for this, but that's misuse, and makes well 187behaved modules like Data::Dumper look bad. The flag is useless for this 188purpose, because it's off when an 8 bit encoding (by default ISO-8859-1) is 189used to store the string. 190 191This is something you, the programmer, has to keep track of; sorry. You could 192consider adopting a kind of "Hungarian notation" to help with this. 193 194=head2 How do I convert from encoding FOO to encoding BAR? 195 196By first converting the FOO-encoded byte string to a text string, and then the 197text string to a BAR-encoded byte string: 198 199 my $text_string = decode('FOO', $foo_string); 200 my $bar_string = encode('BAR', $text_string); 201 202or by skipping the text string part, and going directly from one binary 203encoding to the other: 204 205 use Encode qw(from_to); 206 from_to($string, 'FOO', 'BAR'); # changes contents of $string 207 208or by letting automatic decoding and encoding do all the work: 209 210 open my $foofh, '<:encoding(FOO)', 'example.foo.txt'; 211 open my $barfh, '>:encoding(BAR)', 'example.bar.txt'; 212 print { $barfh } $_ while <$foofh>; 213 214=head2 What are C<decode_utf8> and C<encode_utf8>? 215 216These are alternate syntaxes for C<decode('utf8', ...)> and C<encode('utf8', 217...)>. Do not use these functions for data exchange. Instead use 218C<decode('UTF-8', ...)> and C<encode('UTF-8', ...)>; see 219L</What's the difference between UTF-8 and utf8?> below. 220 221=head2 What is a "wide character"? 222 223This is a term used for characters occupying more than one byte. 224 225The Perl warning "Wide character in ..." is caused by such a character. 226With no specified encoding layer, Perl tries to 227fit things into a single byte. When it can't, it 228emits this warning (if warnings are enabled), and uses UTF-8 encoded data 229instead. 230 231To avoid this warning and to avoid having different output encodings in a single 232stream, always specify an encoding explicitly, for example with a PerlIO layer: 233 234 binmode STDOUT, ":encoding(UTF-8)"; 235 236=head1 INTERNALS 237 238=head2 What is "the UTF8 flag"? 239 240Please, unless you're hacking the internals, or debugging weirdness, don't 241think about the UTF8 flag at all. That means that you very probably shouldn't 242use C<is_utf8>, C<_utf8_on> or C<_utf8_off> at all. 243 244The UTF8 flag, also called SvUTF8, is an internal flag that indicates that the 245current internal representation is UTF-8. Without the flag, it is assumed to be 246ISO-8859-1. Perl converts between these automatically. (Actually Perl usually 247assumes the representation is ASCII; see L</Why do regex character classes 248sometimes match only in the ASCII range?> above.) 249 250One of Perl's internal formats happens to be UTF-8. Unfortunately, Perl can't 251keep a secret, so everyone knows about this. That is the source of much 252confusion. It's better to pretend that the internal format is some unknown 253encoding, and that you always have to encode and decode explicitly. 254 255=head2 What about the C<use bytes> pragma? 256 257Don't use it. It makes no sense to deal with bytes in a text string, and it 258makes no sense to deal with characters in a byte string. Do the proper 259conversions (by decoding/encoding), and things will work out well: you get 260character counts for decoded data, and byte counts for encoded data. 261 262C<use bytes> is usually a failed attempt to do something useful. Just forget 263about it. 264 265=head2 What about the C<use encoding> pragma? 266 267Don't use it. Unfortunately, it assumes that the programmer's environment and 268that of the user will use the same encoding. It will use the same encoding for 269the source code and for STDIN and STDOUT. When a program is copied to another 270machine, the source code does not change, but the STDIO environment might. 271 272If you need non-ASCII characters in your source code, make it a UTF-8 encoded 273file and C<use utf8>. 274 275If you need to set the encoding for STDIN, STDOUT, and STDERR, for example 276based on the user's locale, C<use open>. 277 278=head2 What is the difference between C<:encoding> and C<:utf8>? 279 280Because UTF-8 is one of Perl's internal formats, you can often just skip the 281encoding or decoding step, and manipulate the UTF8 flag directly. 282 283Instead of C<:encoding(UTF-8)>, you can simply use C<:utf8>, which skips the 284encoding step if the data was already represented as UTF8 internally. This is 285widely accepted as good behavior when you're writing, but it can be dangerous 286when reading, because it causes internal inconsistency when you have invalid 287byte sequences. Using C<:utf8> for input can sometimes result in security 288breaches, so please use C<:encoding(UTF-8)> instead. 289 290Instead of C<decode> and C<encode>, you could use C<_utf8_on> and C<_utf8_off>, 291but this is considered bad style. Especially C<_utf8_on> can be dangerous, for 292the same reason that C<:utf8> can. 293 294There are some shortcuts for oneliners; 295see L<-C in perlrun|perlrun/-C [numberE<sol>list]>. 296 297=head2 What's the difference between C<UTF-8> and C<utf8>? 298 299C<UTF-8> is the official standard. C<utf8> is Perl's way of being liberal in 300what it accepts. If you have to communicate with things that aren't so liberal, 301you may want to consider using C<UTF-8>. If you have to communicate with things 302that are too liberal, you may have to use C<utf8>. The full explanation is in 303L<Encode/"UTF-8 vs. utf8 vs. UTF8">. 304 305C<UTF-8> is internally known as C<utf-8-strict>. The tutorial uses UTF-8 306consistently, even where utf8 is actually used internally, because the 307distinction can be hard to make, and is mostly irrelevant. 308 309For example, utf8 can be used for code points that don't exist in Unicode, like 3109999999, but if you encode that to UTF-8, you get a substitution character (by 311default; see L<Encode/"Handling Malformed Data"> for more ways of dealing with 312this.) 313 314Okay, if you insist: the "internal format" is utf8, not UTF-8. (When it's not 315some other encoding.) 316 317=head2 I lost track; what encoding is the internal format really? 318 319It's good that you lost track, because you shouldn't depend on the internal 320format being any specific encoding. But since you asked: by default, the 321internal format is either ISO-8859-1 (latin-1), or utf8, depending on the 322history of the string. On EBCDIC platforms, this may be different even. 323 324Perl knows how it stored the string internally, and will use that knowledge 325when you C<encode>. In other words: don't try to find out what the internal 326encoding for a certain string is, but instead just encode it into the encoding 327that you want. 328 329=head1 AUTHOR 330 331Juerd Waalboer <#####@juerd.nl> 332 333=head1 SEE ALSO 334 335L<perlunicode>, L<perluniintro>, L<Encode> 336 337