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   1  =head1 NAME
   2  
   3  perlunifaq - Perl Unicode FAQ
   4  
   5  =head1 Q and A
   6  
   7  This is a list of questions and answers about Unicode in Perl, intended to be
   8  read after L<perlunitut>.
   9  
  10  =head2 perlunitut isn't really a Unicode tutorial, is it?
  11  
  12  No, and this isn't really a Unicode FAQ.
  13  
  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.
  18  
  19  =head2 What character encodings does Perl support?
  20  
  21  To 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  
  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>.
  29  
  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.
  33  
  34  =head2 What about binary data, like images?
  35  
  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
  38  systems.)
  39  
  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
  43  don't encode?".
  44  
  45  =head2 When should I decode or encode?
  46  
  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.
  50  
  51  =head2 What if I don't decode?
  52  
  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<&amp;gt;>), or double URI encoding
  59  (C<%253E>).
  60  
  61  This silent implicit decoding is known as "upgrading". That may sound
  62  positive, but it's best to avoid it.
  63  
  64  =head2 What if I don't encode?
  65  
  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
  68  friendly warning:
  69  
  70      Wide character in print at example.pl line 2.
  71  
  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
  76  thought this through.
  77  
  78  =head2 Is there a way to automatically decode or encode?
  79  
  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.
  84  
  85  You can provide this layer when C<open>ing the file:
  86  
  87      open my $fh, '>:encoding(UTF-8)', $filename;  # auto encoding on write
  88      open my $fh, '<:encoding(UTF-8)', $filename;  # auto decoding on read
  89  
  90  Or if you already have an open filehandle:
  91  
  92      binmode $fh, ':encoding(UTF-8)';
  93  
  94  Some database drivers for DBI can also automatically encode and decode, but
  95  that is sometimes limited to the UTF-8 encoding.
  96  
  97  =head2 What if I don't know which encoding was used?
  98  
  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.)
 101  
 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
 104  way they should.
 105  
 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.
 108  
 109  =head2 Can I use Unicode in my Perl sources?
 110  
 111  Yes, you can! If your sources are UTF-8 encoded, you can indicate that with the
 112  C<use utf8> pragma.
 113  
 114      use utf8;
 115  
 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.
 120  
 121  =head2 Data::Dumper doesn't restore the UTF8 flag; is it broken?
 122  
 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.
 127  
 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
 132  UTF-8. 
 133  
 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.
 136  
 137  =head2 Why do regex character classes sometimes match only in the ASCII range?
 138  
 139  =head2 Why do some characters not uppercase or lowercase correctly?
 140  
 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.
 145  
 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:...)>,
 148  C</[[:posix:]]/>.
 149  
 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
 152  already upgraded.
 153  
 154  For a more detailed discussion, see L<Unicode::Semantics> on CPAN.
 155  
 156  =head2 How can I determine if a string is a text string or a binary string?
 157  
 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.
 162  
 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.
 165  
 166  =head2 How do I convert from encoding FOO to encoding BAR?
 167  
 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:
 170  
 171      my $text_string = decode('FOO', $foo_string);
 172      my $bar_string  = encode('BAR', $text_string);
 173  
 174  or by skipping the text string part, and going directly from one binary
 175  encoding to the other:
 176  
 177      use Encode qw(from_to);
 178      from_to($string, 'FOO', 'BAR');  # changes contents of $string
 179  
 180  or by letting automatic decoding and encoding do all the work:
 181  
 182      open my $foofh, '<:encoding(FOO)', 'example.foo.txt';
 183      open my $barfh, '>:encoding(BAR)', 'example.bar.txt';
 184      print { $barfh } $_ while <$foofh>;
 185  
 186  =head2 What are C<decode_utf8> and C<encode_utf8>?
 187  
 188  These are alternate syntaxes for C<decode('utf8', ...)> and C<encode('utf8',
 189  ...)>.
 190  
 191  =head2 What is a "wide character"?
 192  
 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.
 196  
 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
 201  instead.
 202  
 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:
 205  
 206      binmode STDOUT, ":encoding(UTF-8)";
 207  
 208  =head1 INTERNALS
 209  
 210  =head2 What is "the UTF8 flag"?
 211  
 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.
 215  
 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.
 219  
 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.
 224  
 225  =head2 What about the C<use bytes> pragma?
 226  
 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.
 231  
 232  C<use bytes> is usually a failed attempt to do something useful. Just forget
 233  about it.
 234  
 235  =head2 What about the C<use encoding> pragma?
 236  
 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.
 241  
 242  If you need non-ASCII characters in your source code, make it a UTF-8 encoded
 243  file and C<use utf8>.
 244  
 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>.
 247  
 248  =head2 What is the difference between C<:encoding> and C<:utf8>?
 249  
 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.
 252  
 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.
 259  
 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.
 263  
 264  There are some shortcuts for oneliners; see C<-C> in L<perlrun>.
 265  
 266  =head2 What's the difference between C<UTF-8> and C<utf8>?
 267  
 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
 272  L<Encode>.
 273  
 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.
 277  
 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
 281  this.)
 282  
 283  Okay, if you insist: the "internal format" is utf8, not UTF-8. (When it's not
 284  some other encoding.)
 285  
 286  =head2 I lost track; what encoding is the internal format really?
 287  
 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.
 292  
 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
 296  that you want.
 297  
 298  =head1 AUTHOR
 299  
 300  Juerd Waalboer <#####@juerd.nl>
 301  
 302  =head1 SEE ALSO
 303  
 304  L<perlunicode>, L<perluniintro>, L<Encode>
 305