lib/perl5/5.8.8/msys/Encode.pm

   1 #
   2 # $Id: Encode.pm,v 2.12 2005/09/08 14:17:17 dankogai Exp dankogai $
   3 #
   4 package Encode;
   5 use strict;
   6 our $VERSION = sprintf "%d.%02d", q$Revision: 2.12 $ =~ /(\d+)/g;
   7 sub DEBUG () { 0 }
   8 use XSLoader ();
   9 XSLoader::load(__PACKAGE__, $VERSION);
  10
  11 require Exporter;
  12 use base qw/Exporter/;
  13
  14 # Public, encouraged API is exported by default
  15
  16 our @EXPORT = qw(
  17   decode  decode_utf8  encode  encode_utf8
  18   encodings  find_encoding clone_encoding
  19 );
  20
  21 our @FB_FLAGS  = qw(DIE_ON_ERR WARN_ON_ERR RETURN_ON_ERR LEAVE_SRC
  22                     PERLQQ HTMLCREF XMLCREF STOP_AT_PARTIAL);
  23 our @FB_CONSTS = qw(FB_DEFAULT FB_CROAK FB_QUIET FB_WARN
  24                     FB_PERLQQ FB_HTMLCREF FB_XMLCREF);
  25
  26 our @EXPORT_OK =
  27     (
  28      qw(
  29        _utf8_off _utf8_on define_encoding from_to is_16bit is_8bit
  30        is_utf8 perlio_ok resolve_alias utf8_downgrade utf8_upgrade
  31       ),
  32      @FB_FLAGS, @FB_CONSTS,
  33     );
  34
  35 our %EXPORT_TAGS =
  36     (
  37      all          =>  [ @EXPORT, @EXPORT_OK ],
  38      fallbacks    =>  [ @FB_CONSTS ],
  39      fallback_all =>  [ @FB_CONSTS, @FB_FLAGS ],
  40     );
  41
  42 # Documentation moved after __END__ for speed - NI-S
  43
  44 our $ON_EBCDIC = (ord("A") == 193);
  45
  46 use Encode::Alias;
  47
  48 # Make a %Encoding package variable to allow a certain amount of cheating
  49 our %Encoding;
  50 our %ExtModule;
  51 require Encode::Config;
  52 eval { require Encode::ConfigLocal };
  53
  54 sub encodings
  55 {
  56     my $class = shift;
  57     my %enc;
  58     if (@_ and $_[0] eq ":all"){
  59         %enc = ( %Encoding, %ExtModule );
  60     }else{
  61         %enc = %Encoding;
  62         for my $mod (map {m/::/o ? $_ : "Encode::$_" } @_){
  63             DEBUG and warn $mod;
  64             for my $enc (keys %ExtModule){
  65                 $ExtModule{$enc} eq $mod and $enc{$enc} = $mod;
  66             }
  67         }
  68     }
  69     return
  70         sort { lc $a cmp lc $b }
  71              grep {!/^(?:Internal|Unicode|Guess)$/o} keys %enc;
  72 }
  73
  74 sub perlio_ok{
  75     my $obj = ref($_[0]) ? $_[0] : find_encoding($_[0]);
  76     $obj->can("perlio_ok") and return $obj->perlio_ok();
  77     return 0; # safety net
  78 }
  79
  80 sub define_encoding
  81 {
  82     my $obj  = shift;
  83     my $name = shift;
  84     $Encoding{$name} = $obj;
  85     my $lc = lc($name);
  86     define_alias($lc => $obj) unless $lc eq $name;
  87     while (@_){
  88         my $alias = shift;
  89         define_alias($alias, $obj);
  90     }
  91     return $obj;
  92 }
  93
  94 sub getEncoding
  95 {
  96     my ($class, $name, $skip_external) = @_;
  97
  98     ref($name) && $name->can('renew') and return $name;
  99     exists $Encoding{$name} and return $Encoding{$name};
 100     my $lc = lc $name;
 101     exists $Encoding{$lc} and return $Encoding{$lc};
 102
 103     my $oc = $class->find_alias($name);
 104     defined($oc) and return $oc;
 105     $lc ne $name and $oc = $class->find_alias($lc);
 106     defined($oc) and return $oc;
 107
 108     unless ($skip_external)
 109     {
 110         if (my $mod = $ExtModule{$name} || $ExtModule{$lc}){
 111             $mod =~ s,::,/,g ; $mod .= '.pm';
 112             eval{ require $mod; };
 113             exists $Encoding{$name} and return $Encoding{$name};
 114         }
 115     }
 116     return;
 117 }
 118
 119 sub find_encoding($;$)
 120 {
 121     my ($name, $skip_external) = @_;
 122     return __PACKAGE__->getEncoding($name,$skip_external);
 123 }
 124
 125 sub resolve_alias($){
 126     my $obj = find_encoding(shift);
 127     defined $obj and return $obj->name;
 128     return;
 129 }
 130
 131 sub clone_encoding($){
 132     my $obj = find_encoding(shift);
 133     ref $obj or return;
 134     eval { require Storable };
 135     $@ and return;
 136     return Storable::dclone($obj);
 137 }
 138
 139 sub encode($$;$)
 140 {
 141     my ($name, $string, $check) = @_;
 142     return undef unless defined $string;
 143     $string .= '' if ref $string; # stringify;
 144     $check ||=0;
 145     my $enc = find_encoding($name);
 146     unless(defined $enc){
 147         require Carp;
 148         Carp::croak("Unknown encoding '$name'");
 149     }
 150     my $octets = $enc->encode($string,$check);
 151     $_[1] = $string if $check and !($check & LEAVE_SRC());
 152     return $octets;
 153 }
 154
 155 sub decode($$;$)
 156 {
 157     my ($name,$octets,$check) = @_;
 158     return undef unless defined $octets;
 159     $octets .= '' if ref $octets;
 160     $check ||=0;
 161     my $enc = find_encoding($name);
 162     unless(defined $enc){
 163         require Carp;
 164         Carp::croak("Unknown encoding '$name'");
 165     }
 166     my $string = $enc->decode($octets,$check);
 167     $_[1] = $octets if $check and !($check & LEAVE_SRC());
 168     return $string;
 169 }
 170
 171 sub from_to($$$;$)
 172 {
 173     my ($string,$from,$to,$check) = @_;
 174     return undef unless defined $string;
 175     $check ||=0;
 176     my $f = find_encoding($from);
 177     unless (defined $f){
 178         require Carp;
 179         Carp::croak("Unknown encoding '$from'");
 180     }
 181     my $t = find_encoding($to);
 182     unless (defined $t){
 183         require Carp;
 184         Carp::croak("Unknown encoding '$to'");
 185     }
 186     my $uni = $f->decode($string,$check);
 187     return undef if ($check && length($string));
 188     $string =  $t->encode($uni,$check);
 189     return undef if ($check && length($uni));
 190     return defined($_[0] = $string) ? length($string) : undef ;
 191 }
 192
 193 sub encode_utf8($)
 194 {
 195     my ($str) = @_;
 196     utf8::encode($str);
 197     return $str;
 198 }
 199
 200 sub decode_utf8($;$)
 201 {
 202     my ($str, $check) = @_;
 203     if ($check){
 204         return decode("utf8", $str, $check);
 205     }else{
 206         return decode("utf8", $str);
 207         return $str;
 208     }
 209 }
 210
 211 predefine_encodings(1);
 212
 213 #
 214 # This is to restore %Encoding if really needed;
 215 #
 216
 217 sub predefine_encodings{
 218     use Encode::Encoding;
 219     no warnings 'redefine';
 220     my $use_xs = shift;
 221     if ($ON_EBCDIC) {
 222         # was in Encode::UTF_EBCDIC
 223         package Encode::UTF_EBCDIC;
 224         push @Encode::UTF_EBCDIC::ISA, 'Encode::Encoding';
 225         *decode = sub{
 226             my ($obj,$str,$chk) = @_;
 227             my $res = '';
 228             for (my $i = 0; $i < length($str); $i++) {
 229                 $res .=
 230                     chr(utf8::unicode_to_native(ord(substr($str,$i,1))));
 231             }
 232             $_[1] = '' if $chk;
 233             return $res;
 234         };
 235         *encode = sub{
 236             my ($obj,$str,$chk) = @_;
 237             my $res = '';
 238             for (my $i = 0; $i < length($str); $i++) {
 239                 $res .=
 240                     chr(utf8::native_to_unicode(ord(substr($str,$i,1))));
 241             }
 242             $_[1] = '' if $chk;
 243             return $res;
 244         };
 245         $Encode::Encoding{Unicode} =
 246             bless {Name => "UTF_EBCDIC"} => "Encode::UTF_EBCDIC";
 247     } else {
 248         package Encode::Internal;
 249         push @Encode::Internal::ISA, 'Encode::Encoding';
 250         *decode = sub{
 251             my ($obj,$str,$chk) = @_;
 252             utf8::upgrade($str);
 253             $_[1] = '' if $chk;
 254             return $str;
 255         };
 256         *encode = \&decode;
 257         $Encode::Encoding{Unicode} =
 258             bless {Name => "Internal"} => "Encode::Internal";
 259     }
 260
 261     {
 262         # was in Encode::utf8
 263         package Encode::utf8;
 264         push @Encode::utf8::ISA, 'Encode::Encoding';
 265         #
 266         if ($use_xs){
 267             Encode::DEBUG and warn __PACKAGE__, " XS on";
 268             *decode = \&decode_xs;
 269             *encode = \&encode_xs;
 270         }else{
 271             Encode::DEBUG and warn __PACKAGE__, " XS off";
 272             *decode = sub{
 273                 my ($obj,$octets,$chk) = @_;
 274                 my $str = Encode::decode_utf8($octets);
 275                 if (defined $str) {
 276                     $_[1] = '' if $chk;
 277                     return $str;
 278                 }
 279                 return undef;
 280             };
 281             *encode = sub {
 282                 my ($obj,$string,$chk) = @_;
 283                 my $octets = Encode::encode_utf8($string);
 284                 $_[1] = '' if $chk;
 285                 return $octets;
 286             };
 287         }
 288         *cat_decode = sub{ # ($obj, $dst, $src, $pos, $trm, $chk)
 289             my ($obj, undef, undef, $pos, $trm) = @_; # currently ignores $chk
 290             my ($rdst, $rsrc, $rpos) = \@_[1,2,3];
 291             use bytes;
 292             if ((my $npos = index($$rsrc, $trm, $pos)) >= 0) {
 293                 $$rdst .= substr($$rsrc, $pos, $npos - $pos + length($trm));
 294                 $$rpos = $npos + length($trm);
 295                 return 1;
 296             }
 297             $$rdst .= substr($$rsrc, $pos);
 298             $$rpos = length($$rsrc);
 299             return '';
 300         };
 301         $Encode::Encoding{utf8} =
 302             bless {Name => "utf8"} => "Encode::utf8";
 303         $Encode::Encoding{"utf-8-strict"} =
 304             bless {Name => "utf-8-strict", strict_utf8 => 1 } => "Encode::utf8";
 305     }
 306 }
 307
 308 1;
 309
 310 __END__
 311
 312 =head1 NAME
 313
 314 Encode - character encodings
 315
 316 =head1 SYNOPSIS
 317
 318     use Encode;
 319
 320 =head2 Table of Contents
 321
 322 Encode consists of a collection of modules whose details are too big
 323 to fit in one document.  This POD itself explains the top-level APIs
 324 and general topics at a glance.  For other topics and more details,
 325 see the PODs below:
 326
 327   Name                          Description
 328   --------------------------------------------------------
 329   Encode::Alias         Alias definitions to encodings
 330   Encode::Encoding      Encode Implementation Base Class
 331   Encode::Supported     List of Supported Encodings
 332   Encode::CN            Simplified Chinese Encodings
 333   Encode::JP            Japanese Encodings
 334   Encode::KR            Korean Encodings
 335   Encode::TW            Traditional Chinese Encodings
 336   --------------------------------------------------------
 337
 338 =head1 DESCRIPTION
 339
 340 The C<Encode> module provides the interfaces between Perl's strings
 341 and the rest of the system.  Perl strings are sequences of
 342 B<characters>.
 343
 344 The repertoire of characters that Perl can represent is at least that
 345 defined by the Unicode Consortium. On most platforms the ordinal
 346 values of the characters (as returned by C<ord(ch)>) is the "Unicode
 347 codepoint" for the character (the exceptions are those platforms where
 348 the legacy encoding is some variant of EBCDIC rather than a super-set
 349 of ASCII - see L<perlebcdic>).
 350
 351 Traditionally, computer data has been moved around in 8-bit chunks
 352 often called "bytes". These chunks are also known as "octets" in
 353 networking standards. Perl is widely used to manipulate data of many
 354 types - not only strings of characters representing human or computer
 355 languages but also "binary" data being the machine's representation of
 356 numbers, pixels in an image - or just about anything.
 357
 358 When Perl is processing "binary data", the programmer wants Perl to
 359 process "sequences of bytes". This is not a problem for Perl - as a
 360 byte has 256 possible values, it easily fits in Perl's much larger
 361 "logical character".
 362
 363 =head2 TERMINOLOGY
 364
 365 =over 2
 366
 367 =item *
 368
 369 I<character>: a character in the range 0..(2**32-1) (or more).
 370 (What Perl's strings are made of.)
 371
 372 =item *
 373
 374 I<byte>: a character in the range 0..255
 375 (A special case of a Perl character.)
 376
 377 =item *
 378
 379 I<octet>: 8 bits of data, with ordinal values 0..255
 380 (Term for bytes passed to or from a non-Perl context, e.g. a disk file.)
 381
 382 =back
 383
 384 =head1 PERL ENCODING API
 385
 386 =over 2
 387
 388 =item $octets  = encode(ENCODING, $string [, CHECK])
 389
 390 Encodes a string from Perl's internal form into I<ENCODING> and returns
 391 a sequence of octets.  ENCODING can be either a canonical name or
 392 an alias.  For encoding names and aliases, see L</"Defining Aliases">.
 393 For CHECK, see L</"Handling Malformed Data">.
 394
 395 For example, to convert a string from Perl's internal format to
 396 iso-8859-1 (also known as Latin1),
 397
 398   $octets = encode("iso-8859-1", $string);
 399
 400 B<CAVEAT>: When you run C<$octets = encode("utf8", $string)>, then $octets
 401 B<may not be equal to> $string.  Though they both contain the same data, the utf8 flag
 402 for $octets is B<always> off.  When you encode anything, utf8 flag of
 403 the result is always off, even when it contains completely valid utf8
 404 string. See L</"The UTF-8 flag"> below.
 405
 406 If the $string is C<undef> then C<undef> is returned.
 407
 408 =item $string = decode(ENCODING, $octets [, CHECK])
 409
 410 Decodes a sequence of octets assumed to be in I<ENCODING> into Perl's
 411 internal form and returns the resulting string.  As in encode(),
 412 ENCODING can be either a canonical name or an alias. For encoding names
 413 and aliases, see L</"Defining Aliases">.  For CHECK, see
 414 L</"Handling Malformed Data">.
 415
 416 For example, to convert ISO-8859-1 data to a string in Perl's internal format:
 417
 418   $string = decode("iso-8859-1", $octets);
 419
 420 B<CAVEAT>: When you run C<$string = decode("utf8", $octets)>, then $string
 421 B<may not be equal to> $octets.  Though they both contain the same data,
 422 the utf8 flag for $string is on unless $octets entirely consists of
 423 ASCII data (or EBCDIC on EBCDIC machines).  See L</"The UTF-8 flag">
 424 below.
 425
 426 If the $string is C<undef> then C<undef> is returned.
 427
 428 =item [$length =] from_to($octets, FROM_ENC, TO_ENC [, CHECK])
 429
 430 Converts B<in-place> data between two encodings. The data in $octets
 431 must be encoded as octets and not as characters in Perl's internal
 432 format. For example, to convert ISO-8859-1 data to Microsoft's CP1250
 433 encoding:
 434
 435   from_to($octets, "iso-8859-1", "cp1250");
 436
 437 and to convert it back:
 438
 439   from_to($octets, "cp1250", "iso-8859-1");
 440
 441 Note that because the conversion happens in place, the data to be
 442 converted cannot be a string constant; it must be a scalar variable.
 443
 444 from_to() returns the length of the converted string in octets on
 445 success, I<undef> on error.
 446
 447 B<CAVEAT>: The following operations look the same but are not quite so;
 448
 449   from_to($data, "iso-8859-1", "utf8"); #1
 450   $data = decode("iso-8859-1", $data);  #2
 451
 452 Both #1 and #2 make $data consist of a completely valid UTF-8 string
 453 but only #2 turns utf8 flag on.  #1 is equivalent to
 454
 455   $data = encode("utf8", decode("iso-8859-1", $data));
 456
 457 See L</"The UTF-8 flag"> below.
 458
 459 =item $octets = encode_utf8($string);
 460
 461 Equivalent to C<$octets = encode("utf8", $string);> The characters
 462 that comprise $string are encoded in Perl's internal format and the
 463 result is returned as a sequence of octets. All possible
 464 characters have a UTF-8 representation so this function cannot fail.
 465
 466
 467 =item $string = decode_utf8($octets [, CHECK]);
 468
 469 equivalent to C<$string = decode("utf8", $octets [, CHECK])>.
 470 The sequence of octets represented by
 471 $octets is decoded from UTF-8 into a sequence of logical
 472 characters. Not all sequences of octets form valid UTF-8 encodings, so
 473 it is possible for this call to fail.  For CHECK, see
 474 L</"Handling Malformed Data">.
 475
 476 =back
 477
 478 =head2 Listing available encodings
 479
 480   use Encode;
 481   @list = Encode->encodings();
 482
 483 Returns a list of the canonical names of the available encodings that
 484 are loaded.  To get a list of all available encodings including the
 485 ones that are not loaded yet, say
 486
 487   @all_encodings = Encode->encodings(":all");
 488
 489 Or you can give the name of a specific module.
 490
 491   @with_jp = Encode->encodings("Encode::JP");
 492
 493 When "::" is not in the name, "Encode::" is assumed.
 494
 495   @ebcdic = Encode->encodings("EBCDIC");
 496
 497 To find out in detail which encodings are supported by this package,
 498 see L<Encode::Supported>.
 499
 500 =head2 Defining Aliases
 501
 502 To add a new alias to a given encoding, use:
 503
 504   use Encode;
 505   use Encode::Alias;
 506   define_alias(newName => ENCODING);
 507
 508 After that, newName can be used as an alias for ENCODING.
 509 ENCODING may be either the name of an encoding or an
 510 I<encoding object>
 511
 512 But before you do so, make sure the alias is nonexistent with
 513 C<resolve_alias()>, which returns the canonical name thereof.
 514 i.e.
 515
 516   Encode::resolve_alias("latin1") eq "iso-8859-1" # true
 517   Encode::resolve_alias("iso-8859-12")   # false; nonexistent
 518   Encode::resolve_alias($name) eq $name  # true if $name is canonical
 519
 520 resolve_alias() does not need C<use Encode::Alias>; it can be
 521 exported via C<use Encode qw(resolve_alias)>.
 522
 523 See L<Encode::Alias> for details.
 524
 525 =head1 Encoding via PerlIO
 526
 527 If your perl supports I<PerlIO> (which is the default), you can use a PerlIO layer to decode
 528 and encode directly via a filehandle.  The following two examples
 529 are totally identical in their functionality.
 530
 531   # via PerlIO
 532   open my $in,  "<:encoding(shiftjis)", $infile  or die;
 533   open my $out, ">:encoding(euc-jp)",   $outfile or die;
 534   while(<$in>){ print $out $_; }
 535
 536   # via from_to
 537   open my $in,  "<", $infile  or die;
 538   open my $out, ">", $outfile or die;
 539   while(<$in>){
 540     from_to($_, "shiftjis", "euc-jp", 1);
 541     print $out $_;
 542   }
 543
 544 Unfortunately, it may be that encodings are PerlIO-savvy.  You can check
 545 if your encoding is supported by PerlIO by calling the C<perlio_ok>
 546 method.
 547
 548   Encode::perlio_ok("hz");             # False
 549   find_encoding("euc-cn")->perlio_ok;  # True where PerlIO is available
 550
 551   use Encode qw(perlio_ok);            # exported upon request
 552   perlio_ok("euc-jp")
 553
 554 Fortunately, all encodings that come with Encode core are PerlIO-savvy
 555 except for hz and ISO-2022-kr.  For gory details, see
 556 L<Encode::Encoding> and L<Encode::PerlIO>.
 557
 558 =head1 Handling Malformed Data
 559
 560 The optional I<CHECK> argument tells Encode what to do when it
 561 encounters malformed data.  Without CHECK, Encode::FB_DEFAULT ( == 0 )
 562 is assumed.
 563
 564 As of version 2.12 Encode supports coderef values for CHECK.  See below.
 565
 566 =over 2
 567
 568 =item B<NOTE:> Not all encoding support this feature
 569
 570 Some encodings ignore I<CHECK> argument.  For example,
 571 L<Encode::Unicode> ignores I<CHECK> and it always croaks on error.
 572
 573 =back
 574
 575 Now here is the list of I<CHECK> values available
 576
 577 =over 2
 578
 579 =item I<CHECK> = Encode::FB_DEFAULT ( == 0)
 580
 581 If I<CHECK> is 0, (en|de)code will put a I<substitution character> in
 582 place of a malformed character.  When you encode, E<lt>subcharE<gt>
 583 will be used.  When you decode the code point C<0xFFFD> is used.  If
 584 the data is supposed to be UTF-8, an optional lexical warning
 585 (category utf8) is given.
 586
 587 =item I<CHECK> = Encode::FB_CROAK ( == 1)
 588
 589 If I<CHECK> is 1, methods will die on error immediately with an error
 590 message.  Therefore, when I<CHECK> is set to 1,  you should trap the
 591 error with eval{} unless you really want to let it die.
 592
 593 =item I<CHECK> = Encode::FB_QUIET
 594
 595 If I<CHECK> is set to Encode::FB_QUIET, (en|de)code will immediately
 596 return the portion of the data that has been processed so far when an
 597 error occurs. The data argument will be overwritten with everything
 598 after that point (that is, the unprocessed part of data).  This is
 599 handy when you have to call decode repeatedly in the case where your
 600 source data may contain partial multi-byte character sequences,
 601 (i.e. you are reading with a fixed-width buffer). Here is a sample
 602 code that does exactly this:
 603
 604   my $buffer = ''; my $string = '';
 605   while(read $fh, $buffer, 256, length($buffer)){
 606     $string .= decode($encoding, $buffer, Encode::FB_QUIET);
 607     # $buffer now contains the unprocessed partial character
 608   }
 609
 610 =item I<CHECK> = Encode::FB_WARN
 611
 612 This is the same as above, except that it warns on error.  Handy when
 613 you are debugging the mode above.
 614
 615 =item perlqq mode (I<CHECK> = Encode::FB_PERLQQ)
 616
 617 =item HTML charref mode (I<CHECK> = Encode::FB_HTMLCREF)
 618
 619 =item XML charref mode (I<CHECK> = Encode::FB_XMLCREF)
 620
 621 For encodings that are implemented by Encode::XS, CHECK ==
 622 Encode::FB_PERLQQ turns (en|de)code into C<perlqq> fallback mode.
 623
 624 When you decode, C<\xI<HH>> will be inserted for a malformed character,
 625 where I<HH> is the hex representation of the octet  that could not be
 626 decoded to utf8.  And when you encode, C<\x{I<HHHH>}> will be inserted,
 627 where I<HHHH> is the Unicode ID of the character that cannot be found
 628 in the character repertoire of the encoding.
 629
 630 HTML/XML character reference modes are about the same, in place of
 631 C<\x{I<HHHH>}>, HTML uses C<&#I<NNN>;> where I<NNN> is a decimal number and
 632 XML uses C<&#xI<HHHH>;> where I<HHHH> is the hexadecimal number.
 633
 634 In Encode 2.10 or later, C<LEAVE_SRC> is also implied.
 635
 636 =item The bitmask
 637
 638 These modes are actually set via a bitmask.  Here is how the FB_XX
 639 constants are laid out.  You can import the FB_XX constants via
 640 C<use Encode qw(:fallbacks)>; you can import the generic bitmask
 641 constants via C<use Encode qw(:fallback_all)>.
 642
 643                      FB_DEFAULT FB_CROAK FB_QUIET FB_WARN  FB_PERLQQ
 644  DIE_ON_ERR    0x0001             X
 645  WARN_ON_ERR   0x0002                               X
 646  RETURN_ON_ERR 0x0004                      X        X
 647  LEAVE_SRC     0x0008                                        X
 648  PERLQQ        0x0100                                        X
 649  HTMLCREF      0x0200
 650  XMLCREF       0x0400
 651
 652 =back
 653
 654 =head2 coderef for CHECK
 655
 656 As of Encode 2.12 CHECK can also be a code reference which takes the
 657 ord value of unmapped caharacter as an argument and returns a string
 658 that represents the fallback character.  For instance,
 659
 660   $ascii = encode("ascii", $utf8, sub{ sprintf "<U+%04X>", shift });
 661
 662 Acts like FB_PERLQQ but E<lt>U+I<XXXX>E<gt> is used instead of
 663 \x{I<XXXX>}.
 664
 665 =head1 Defining Encodings
 666
 667 To define a new encoding, use:
 668
 669     use Encode qw(define_encoding);
 670     define_encoding($object, 'canonicalName' [, alias...]);
 671
 672 I<canonicalName> will be associated with I<$object>.  The object
 673 should provide the interface described in L<Encode::Encoding>.
 674 If more than two arguments are provided then additional
 675 arguments are taken as aliases for I<$object>.
 676
 677 See L<Encode::Encoding> for more details.
 678
 679 =head1 The UTF-8 flag
 680
 681 Before the introduction of utf8 support in perl, The C<eq> operator
 682 just compared the strings represented by two scalars. Beginning with
 683 perl 5.8, C<eq> compares two strings with simultaneous consideration
 684 of I<the utf8 flag>. To explain why we made it so, I will quote page
 685 402 of C<Programming Perl, 3rd ed.>
 686
 687 =over 2
 688
 689 =item Goal #1:
 690
 691 Old byte-oriented programs should not spontaneously break on the old
 692 byte-oriented data they used to work on.
 693
 694 =item Goal #2:
 695
 696 Old byte-oriented programs should magically start working on the new
 697 character-oriented data when appropriate.
 698
 699 =item Goal #3:
 700
 701 Programs should run just as fast in the new character-oriented mode
 702 as in the old byte-oriented mode.
 703
 704 =item Goal #4:
 705
 706 Perl should remain one language, rather than forking into a
 707 byte-oriented Perl and a character-oriented Perl.
 708
 709 =back
 710
 711 Back when C<Programming Perl, 3rd ed.> was written, not even Perl 5.6.0
 712 was born and many features documented in the book remained
 713 unimplemented for a long time.  Perl 5.8 corrected this and the introduction
 714 of the UTF-8 flag is one of them.  You can think of this perl notion as of a
 715 byte-oriented mode (utf8 flag off) and a character-oriented mode (utf8
 716 flag on).
 717
 718 Here is how Encode takes care of the utf8 flag.
 719
 720 =over 2
 721
 722 =item *
 723
 724 When you encode, the resulting utf8 flag is always off.
 725
 726 =item *
 727
 728 When you decode, the resulting utf8 flag is on unless you can
 729 unambiguously represent data.  Here is the definition of
 730 dis-ambiguity.
 731
 732 After C<$utf8 = decode('foo', $octet);>,
 733
 734   When $octet is...   The utf8 flag in $utf8 is
 735   ---------------------------------------------
 736   In ASCII only (or EBCDIC only)            OFF
 737   In ISO-8859-1                              ON
 738   In any other Encoding                      ON
 739   ---------------------------------------------
 740
 741 As you see, there is one exception, In ASCII.  That way you can assume
 742 Goal #1.  And with Encode Goal #2 is assumed but you still have to be
 743 careful in such cases mentioned in B<CAVEAT> paragraphs.
 744
 745 This utf8 flag is not visible in perl scripts, exactly for the same
 746 reason you cannot (or you I<don't have to>) see if a scalar contains a
 747 string, integer, or floating point number.   But you can still peek
 748 and poke these if you will.  See the section below.
 749
 750 =back
 751
 752 =head2 Messing with Perl's Internals
 753
 754 The following API uses parts of Perl's internals in the current
 755 implementation.  As such, they are efficient but may change.
 756
 757 =over 2
 758
 759 =item is_utf8(STRING [, CHECK])
 760
 761 [INTERNAL] Tests whether the UTF-8 flag is turned on in the STRING.
 762 If CHECK is true, also checks the data in STRING for being well-formed
 763 UTF-8.  Returns true if successful, false otherwise.
 764
 765 As of perl 5.8.1, L<utf8> also has utf8::is_utf8().
 766
 767 =item _utf8_on(STRING)
 768
 769 [INTERNAL] Turns on the UTF-8 flag in STRING.  The data in STRING is
 770 B<not> checked for being well-formed UTF-8.  Do not use unless you
 771 B<know> that the STRING is well-formed UTF-8.  Returns the previous
 772 state of the UTF-8 flag (so please don't treat the return value as
 773 indicating success or failure), or C<undef> if STRING is not a string.
 774
 775 =item _utf8_off(STRING)
 776
 777 [INTERNAL] Turns off the UTF-8 flag in STRING.  Do not use frivolously.
 778 Returns the previous state of the UTF-8 flag (so please don't treat the
 779 return value as indicating success or failure), or C<undef> if STRING is
 780 not a string.
 781
 782 =back
 783
 784 =head1 UTF-8 vs. utf8
 785
 786   ....We now view strings not as sequences of bytes, but as sequences
 787   of numbers in the range 0 .. 2**32-1 (or in the case of 64-bit
 788   computers, 0 .. 2**64-1) -- Programming Perl, 3rd ed.
 789
 790 That has been the perl's notion of UTF-8 but official UTF-8 is more
 791 strict; Its ranges is much narrower (0 .. 10FFFF), some sequences are
 792 not allowed (i.e. Those used in the surrogate pair, 0xFFFE, et al).
 793
 794 Now that is overruled by Larry Wall himself.
 795
 796   From: Larry Wall <larry@wall.org>
 797   Date: December 04, 2004 11:51:58 JST
 798   To: perl-unicode@perl.org
 799   Subject: Re: Make Encode.pm support the real UTF-8
 800   Message-Id: <20041204025158.GA28754@wall.org>
 801
 802   On Fri, Dec 03, 2004 at 10:12:12PM +0000, Tim Bunce wrote:
 803   : I've no problem with 'utf8' being perl's unrestricted uft8 encoding,
 804   : but "UTF-8" is the name of the standard and should give the
 805   : corresponding behaviour.
 806
 807   For what it's worth, that's how I've always kept them straight in my
 808   head.
 809
 810   Also for what it's worth, Perl 6 will mostly default to strict but
 811   make it easy to switch back to lax.
 812
 813   Larry
 814
 815 Do you copy?  As of Perl 5.8.7, B<UTF-8> means strict, official UTF-8
 816 while B<utf8> means liberal, lax, version thereof.  And Encode version
 817 2.10 or later thus groks the difference between C<UTF-8> and C"utf8".
 818
 819   encode("utf8",  "\x{FFFF_FFFF}", 1); # okay
 820   encode("UTF-8", "\x{FFFF_FFFF}", 1); # croaks
 821
 822 C<UTF-8> in Encode is actually a canonical name for C<utf-8-strict>.
 823 Yes, the hyphen between "UTF" and "8" is important.  Without it Encode
 824 goes "liberal"
 825
 826   find_encoding("UTF-8")->name # is 'utf-8-strict'
 827   find_encoding("utf-8")->name # ditto. names are case insensitive
 828   find_encoding("utf8")->name  # ditto. "_" are treated as "-"
 829   find_encoding("UTF8")->name  # is 'utf8'.
 830
 831
 832 =head1 SEE ALSO
 833
 834 L<Encode::Encoding>,
 835 L<Encode::Supported>,
 836 L<Encode::PerlIO>,
 837 L<encoding>,
 838 L<perlebcdic>,
 839 L<perlfunc/open>,
 840 L<perlunicode>,
 841 L<utf8>,
 842 the Perl Unicode Mailing List E<lt>perl-unicode@perl.orgE<gt>
 843
 844 =head1 MAINTAINER
 845
 846 This project was originated by Nick Ing-Simmons and later maintained
 847 by Dan Kogai E<lt>dankogai@dan.co.jpE<gt>.  See AUTHORS for a full
 848 list of people involved.  For any questions, use
 849 E<lt>perl-unicode@perl.orgE<gt> so we can all share.
 850
 851 =cut