PC Professionell 2006 December/PCpro_2006_12.ISO/ossdvd/server/Perl2/site/lib/xml/Parser.pm

Perl POD Document | 2002-04-02 | 31.7 KB | 1,088 lines

# XML::Parser
#
# Copyright (c) 1998-2000 Larry Wall and Clark Cooper
# All rights reserved.
#
# This program is free software; you can redistribute it and/or
# modify it under the same terms as Perl itself.

package XML::Parser;

use Carp;
use IO::File;

BEGIN {
  require XML::Parser::Expat;
  $VERSION = '2.31';
  die "Parser.pm and Expat.pm versions don't match"
    unless $VERSION eq $XML::Parser::Expat::VERSION;
}

use strict;

use vars qw($VERSION %Built_In_Styles $LWP_load_failed);

$LWP_load_failed = 0;

sub new {
  my ($class, %args) = @_;
  my $style = $args{Style};

  my $nonexopt = $args{Non_Expat_Options} ||= {};

  $nonexopt->{Style}             = 1;
  $nonexopt->{Non_Expat_Options} = 1;
  $nonexopt->{Handlers}          = 1;
  $nonexopt->{_HNDL_TYPES}       = 1;
  $nonexopt->{NoLWP}             = 1;

  $args{_HNDL_TYPES} = {%XML::Parser::Expat::Handler_Setters};
  $args{_HNDL_TYPES}->{Init} = 1;
  $args{_HNDL_TYPES}->{Final} = 1;

  $args{Handlers} ||= {};
  my $handlers = $args{Handlers};

  if (defined($style)) {
    my $stylepkg = $style;

    if ($stylepkg !~ /::/) {
      $stylepkg = "\u$style";

      # I'm using the Built_In_Styles hash to define
      # valid internal styles, since a style doesn't
      # need to define any particular Handler to be valid.
      # So I can't check for the existence of a particular sub.

      croak "Undefined style: $style"
    unless defined($Built_In_Styles{$stylepkg});
      $stylepkg = 'XML::Parser::' . $stylepkg;
    }

    my $htype;
    foreach $htype (keys %{$args{_HNDL_TYPES}}) {
      # Handlers explicity given override
      # handlers from the Style package
      unless (defined($handlers->{$htype})) {

    # A handler in the style package must either have
    # exactly the right case as the type name or a
    # completely lower case version of it.

    my $hname = "${stylepkg}::$htype";
    if (defined(&$hname)) {
      $handlers->{$htype} = \&$hname;
      next;
    }

    $hname = "${stylepkg}::\L$htype";
    if (defined(&$hname)) {
      $handlers->{$htype} = \&$hname;
      next;
    }
      }
    }
  }

  unless (defined($handlers->{ExternEnt})
      or defined ($handlers->{ExternEntFin})) {

    if ($args{NoLWP} or $LWP_load_failed) {
      $handlers->{ExternEnt} = \&file_ext_ent_handler;
      $handlers->{ExternEntFin} = \&file_ext_ent_cleanup;
    }
    else {
      # The following just bootstraps the real LWP external entity
      # handler

      $handlers->{ExternEnt} = \&initial_ext_ent_handler;

      # No cleanup function available until LWPExternEnt.pl loaded
    }
  }

  $args{Pkg} ||= caller;
  bless \%args, $class;
}                # End of new

sub setHandlers {
  my ($self, @handler_pairs) = @_;

  croak("Uneven number of arguments to setHandlers method")
    if (int(@handler_pairs) & 1);

  my @ret;
  while (@handler_pairs) {
    my $type = shift @handler_pairs;
    my $handler = shift @handler_pairs;
    unless (defined($self->{_HNDL_TYPES}->{$type})) {
      my @types = sort keys %{$self->{_HNDL_TYPES}};

      croak("Unknown Parser handler type: $type\n Valid types: @types");
    }
    push(@ret, $type, $self->{Handlers}->{$type});
    $self->{Handlers}->{$type} = $handler;
  }

  return @ret;
}                # End of setHandlers

sub parse_start {
  my $self = shift;
  my @expat_options = ();

  my ($key, $val);
  while (($key, $val) = each %{$self}) {
    push (@expat_options, $key, $val)
      unless exists $self->{Non_Expat_Options}->{$key};
  }

  my %handlers = %{$self->{Handlers}};
  my $init = delete $handlers{Init};
  my $final = delete $handlers{Final};

  my $expatnb = new XML::Parser::ExpatNB(@expat_options, @_);
  $expatnb->setHandlers(%handlers);

  &$init($expatnb)
    if defined($init);

  $expatnb->{_State_} = 1;

  $expatnb->{FinalHandler} = $final
    if defined($final);

  return $expatnb;
}

sub parse {
  my $self = shift;
  my $arg  = shift;
  my @expat_options = ();
  my ($key, $val);
  while (($key, $val) = each %{$self}) {
    push(@expat_options, $key, $val)
      unless exists $self->{Non_Expat_Options}->{$key};
  }

  my $expat = new XML::Parser::Expat(@expat_options, @_);
  my %handlers = %{$self->{Handlers}};
  my $init = delete $handlers{Init};
  my $final = delete $handlers{Final};

  $expat->setHandlers(%handlers);

  if ($self->{Base}) {
    $expat->base($self->{Base});
  }

  &$init($expat)
    if defined($init);

  my @result = ();
  my $result;
  eval {
    $result = $expat->parse($arg);
  };
  my $err = $@;
  if ($err) {
    $expat->release;
    die $err;
  }

  if ($result and defined($final)) {
    if (wantarray) {
      @result = &$final($expat);
    }
    else {
      $result = &$final($expat);
    }
  }

  $expat->release;

  return unless defined wantarray;
  return wantarray ? @result : $result;
}                # End of parse

sub parsestring {
  my $self = shift;
  $self->parse(@_);
}                # End of parsestring

sub parsefile {
  my $self = shift;
  my $file = shift;
  local(*FILE);
  open(FILE, $file) or  croak "Couldn't open $file:\n$!";
  binmode(FILE);
  my @ret;
  my $ret;

  $self->{Base} = $file;

  if (wantarray) {
    eval {
      @ret = $self->parse(*FILE, @_);
    };
  }
  else {
    eval {
      $ret = $self->parse(*FILE, @_);
    };
  }
  my $err = $@;
  close(FILE);
  die $err if $err;

  return unless defined wantarray;
  return wantarray ? @ret : $ret;
}                # End of parsefile


sub initial_ext_ent_handler {
  # This just bootstraps in the real lwp_ext_ent_handler which
  # also loads the URI and LWP modules.

  unless ($LWP_load_failed) {
    local($^W) = 0;

    my $stat =
      eval {
    require('XML/Parser/LWPExternEnt.pl');
      };

    if ($stat) {
      $_[0]->setHandlers(ExternEnt    => \&lwp_ext_ent_handler,
             ExternEntFin => \&lwp_ext_ent_cleanup);

      goto &lwp_ext_ent_handler;
    }

    # Failed to load lwp handler, act as if NoLWP

    $LWP_load_failed = 1;

    my $cmsg = "Couldn't load LWP based external entity handler\n";
    $cmsg .= "Switching to file-based external entity handler\n";
    $cmsg .= " (To avoid this message, use NoLWP option to XML::Parser)\n";
    warn($cmsg);
  }

  $_[0]->setHandlers(ExternEnt    => \&file_ext_ent_handler,
             ExternEntFin => \&file_ext_ent_cleanup);
  goto &file_ext_ent_handler;

}  # End initial_ext_ent_handler

sub file_ext_ent_handler {
  my ($xp, $base, $path) = @_;

  # Prepend base only for relative paths

  if (defined($base)
      and not ($path =~ m!^(?:[\\/]|\w+:)!))
    {
      my $newpath = $base;
      $newpath =~ s![^\\/:]*$!$path!;
      $path = $newpath;
    }

  if ($path =~ /^\s*[|>+]/
      or $path =~ /\|\s*$/) {
    $xp->{ErrorMessage}
    .= "System ID ($path) contains Perl IO control characters";
    return undef;
  }

  my $fh = new IO::File($path);
  unless (defined $fh) {
    $xp->{ErrorMessage}
      .= "Failed to open $path:\n$!";
    return undef;
  }

  $xp->{_BaseStack} ||= [];
  $xp->{_FhStack} ||= [];

  push(@{$xp->{_BaseStack}}, $base);
  push(@{$xp->{_FhStack}}, $fh);

  $xp->base($path);

  return $fh;
}  # End file_ext_ent_handler

sub file_ext_ent_cleanup {
  my ($xp) = @_;

  my $fh = pop(@{$xp->{_FhStack}});
  $fh->close;

  my $base = pop(@{$xp->{_BaseStack}});
  $xp->base($base);
}  # End file_ext_ent_cleanup

###################################################################

package XML::Parser::Debug;
$XML::Parser::Built_In_Styles{Debug} = 1;

sub Start {
  my $expat = shift;
  my $tag = shift;
  print STDERR "@{$expat->{Context}} \\\\ (@_)\n";
}

sub End {
  my $expat = shift;
  my $tag = shift;
  print STDERR "@{$expat->{Context}} //\n";
}

sub Char {
  my $expat = shift;
  my $text = shift;
  $text =~ s/([\x80-\xff])/sprintf "#x%X;", ord $1/eg;
  $text =~ s/([\t\n])/sprintf "#%d;", ord $1/eg;
  print STDERR "@{$expat->{Context}} || $text\n";
}

sub Proc {
  my $expat = shift;
  my $target = shift;
  my $text = shift;
  my @foo = @{$expat->{Context}};
  print STDERR "@foo $target($text)\n";
}

###################################################################

package XML::Parser::Subs;
$XML::Parser::Built_In_Styles{Subs} = 1;

sub Start {
  no strict 'refs';
  my $expat = shift;
  my $tag = shift;
  my $sub = $expat->{Pkg} . "::$tag";
  eval { &$sub($expat, $tag, @_) };
}

sub End {
  no strict 'refs';
  my $expat = shift;
  my $tag = shift;
  my $sub = $expat->{Pkg} . "::${tag}_";
  eval { &$sub($expat, $tag) };
}

###################################################################

package XML::Parser::Tree;
$XML::Parser::Built_In_Styles{Tree} = 1;

sub Init {
  my $expat = shift;
  $expat->{Lists} = [];
  $expat->{Curlist} = $expat->{Tree} = [];
}

sub Start {
  my $expat = shift;
  my $tag = shift;
  my $newlist = [ { @_ } ];
  push @{ $expat->{Lists} }, $expat->{Curlist};
  push @{ $expat->{Curlist} }, $tag => $newlist;
  $expat->{Curlist} = $newlist;
}

sub End {
  my $expat = shift;
  my $tag = shift;
  $expat->{Curlist} = pop @{ $expat->{Lists} };
}

sub Char {
  my $expat = shift;
  my $text = shift;
  my $clist = $expat->{Curlist};
  my $pos = $#$clist;

  if ($pos > 0 and $clist->[$pos - 1] eq '0') {
    $clist->[$pos] .= $text;
  } else {
    push @$clist, 0 => $text;
  }
}

sub Final {
  my $expat = shift;
  delete $expat->{Curlist};
  delete $expat->{Lists};
  $expat->{Tree};
}

###################################################################

package XML::Parser::Objects;
$XML::Parser::Built_In_Styles{Objects} = 1;

sub Init {
  my $expat = shift;
  $expat->{Lists} = [];
  $expat->{Curlist} = $expat->{Tree} = [];
}

sub Start {
  my $expat = shift;
  my $tag = shift;
  my $newlist = [ ];
  my $class = "${$expat}{Pkg}::$tag";
  my $newobj = bless { @_, Kids => $newlist }, $class;
  push @{ $expat->{Lists} }, $expat->{Curlist};
  push @{ $expat->{Curlist} }, $newobj;
  $expat->{Curlist} = $newlist;
}

sub End {
  my $expat = shift;
  my $tag = shift;
  $expat->{Curlist} = pop @{ $expat->{Lists} };
}

sub Char {
  my $expat = shift;
  my $text = shift;
  my $class = "${$expat}{Pkg}::Characters";
  my $clist = $expat->{Curlist};
  my $pos = $#$clist;

  if ($pos >= 0 and ref($clist->[$pos]) eq $class) {
    $clist->[$pos]->{Text} .= $text;
  } else {
    push @$clist, bless { Text => $text }, $class;
  }
}

sub Final {
  my $expat = shift;
  delete $expat->{Curlist};
  delete $expat->{Lists};
  $expat->{Tree};
}

################################################################

package XML::Parser::Stream;
$XML::Parser::Built_In_Styles{Stream} = 1;

# This style invented by Tim Bray <[e-mail]>

sub Init {
  no strict 'refs';
  my $expat = shift;
  $expat->{Text} = '';
  my $sub = $expat->{Pkg} ."::StartDocument";
  &$sub($expat)
    if defined(&$sub);
}

sub Start {
  no strict 'refs';
  my $expat = shift;
  my $type = shift;

  doText($expat);
  $_ = "<$type";

  %_ = @_;
  while (@_) {
    $_ .= ' ' . shift() . '="' . shift() . '"';
  }
  $_ .= '>';

  my $sub = $expat->{Pkg} . "::StartTag";
  if (defined(&$sub)) {
    &$sub($expat, $type);
  } else {
    print;
  }
}

sub End {
  no strict 'refs';
  my $expat = shift;
  my $type = shift;

  # Set right context for Text handler
  push(@{$expat->{Context}}, $type);
  doText($expat);
  pop(@{$expat->{Context}});

  $_ = "</$type>";

  my $sub = $expat->{Pkg} . "::EndTag";
  if (defined(&$sub)) {
    &$sub($expat, $type);
  } else {
    print;
  }
}

sub Char {
  my $expat = shift;
  $expat->{Text} .= shift;
}

sub Proc {
  no strict 'refs';
  my $expat = shift;
  my $target = shift;
  my $text = shift;

  doText($expat);

  $_ = "<?$target $text?>";

  my $sub = $expat->{Pkg} . "::PI";
  if (defined(&$sub)) {
    &$sub($expat, $target, $text);
  } else {
    print;
  }
}

sub Final {
  no strict 'refs';
  my $expat = shift;
  my $sub = $expat->{Pkg} . "::EndDocument";
  &$sub($expat)
    if defined(&$sub);
}

sub doText {
  no strict 'refs';
  my $expat = shift;
  $_ = $expat->{Text};

  if (length($_)) {
    my $sub = $expat->{Pkg} . "::Text";
    if (defined(&$sub)) {
      &$sub($expat);
    } else {
      print;
    }

    $expat->{Text} = '';
  }
}

1;

__END__

=head1 NAME

XML::Parser - A perl module for parsing XML documents

=head1 SYNOPSIS

  use XML::Parser;

  $p1 = new XML::Parser(Style => 'Debug');
  $p1->parsefile('REC-xml-19980210.xml');
  $p1->parse('<foo id="me">Hello World</foo>');

  # Alternative
  $p2 = new XML::Parser(Handlers => {Start => \&handle_start,
                     End   => \&handle_end,
                     Char  => \&handle_char});
  $p2->parse($socket);

  # Another alternative
  $p3 = new XML::Parser(ErrorContext => 2);

  $p3->setHandlers(Char    => \&text,
           Default => \&other);

  open(FOO, 'xmlgenerator |');
  $p3->parse(*FOO, ProtocolEncoding => 'ISO-8859-1');
  close(FOO);

  $p3->parsefile('junk.xml', ErrorContext => 3);

=begin man
.ds PI PI

=end man

=head1 DESCRIPTION

This module provides ways to parse XML documents. It is built on top of
L<XML::Parser::Expat>, which is a lower level interface to James Clark's
expat library. Each call to one of the parsing methods creates a new
instance of XML::Parser::Expat which is then used to parse the document.
Expat options may be provided when the XML::Parser object is created.
These options are then passed on to the Expat object on each parse call.
They can also be given as extra arguments to the parse methods, in which
case they override options given at XML::Parser creation time.

The behavior of the parser is controlled either by C<L</Style>> and/or
C<L</Handlers>> options, or by L</setHandlers> method. These all provide
mechanisms for XML::Parser to set the handlers needed by XML::Parser::Expat.
If neither C<Style> nor C<Handlers> are specified, then parsing just
checks the document for being well-formed.

When underlying handlers get called, they receive as their first parameter
the I<Expat> object, not the Parser object.

=head1 METHODS

=over 4

=item new

This is a class method, the constructor for XML::Parser. Options are passed
as keyword value pairs. Recognized options are:

=over 4

=item * Style

This option provides an easy way to create a given style of parser. The
built in styles are: L<"Debug">, L<"Subs">, L<"Tree">, L<"Objects">,
and L<"Stream">.
Custom styles can be provided by giving a full package name containing
at least one '::'. This package should then have subs defined for each
handler it wishes to have installed. See L<"STYLES"> below
for a discussion of each built in style.

=item * Handlers

When provided, this option should be an anonymous hash containing as
keys the type of handler and as values a sub reference to handle that
type of event. All the handlers get passed as their 1st parameter the
instance of expat that is parsing the document. Further details on
handlers can be found in L<"HANDLERS">. Any handler set here
overrides the corresponding handler set with the Style option.

=item * Pkg

Some styles will refer to subs defined in this package. If not provided,
it defaults to the package which called the constructor.

=item * ErrorContext

This is an Expat option. When this option is defined, errors are reported
in context. The value should be the number of lines to show on either side
of the line in which the error occurred.

=item * ProtocolEncoding

This is an Expat option. This sets the protocol encoding name. It defaults
to none. The built-in encodings are: C<UTF-8>, C<ISO-8859-1>, C<UTF-16>, and
C<US-ASCII>. Other encodings may be used if they have encoding maps in one
of the directories in the @Encoding_Path list. Check L<"ENCODINGS"> for
more information on encoding maps. Setting the protocol encoding overrides
any encoding in the XML declaration.

=item * Namespaces

This is an Expat option. If this is set to a true value, then namespace
processing is done during the parse. See L<XML::Parser::Expat/"Namespaces">
for further discussion of namespace processing.

=item * NoExpand

This is an Expat option. Normally, the parser will try to expand references
to entities defined in the internal subset. If this option is set to a true
value, and a default handler is also set, then the default handler will be
called when an entity reference is seen in text. This has no effect if a
default handler has not been registered, and it has no effect on the expansion
of entity references inside attribute values.

=item * Stream_Delimiter

This is an Expat option. It takes a string value. When this string is found
alone on a line while parsing from a stream, then the parse is ended as if it
saw an end of file. The intended use is with a stream of xml documents in a
MIME multipart format. The string should not contain a trailing newline.

=item * ParseParamEnt

This is an Expat option. Unless standalone is set to "yes" in the XML
declaration, setting this to a true value allows the external DTD to be read,
and parameter entities to be parsed and expanded.

=item * NoLWP

This option has no effect if the ExternEnt or ExternEntFin handlers are
directly set. Otherwise, if true, it forces the use of a file based external
entity handler.

=item * Non-Expat-Options

If provided, this should be an anonymous hash whose keys are options that
shouldn't be passed to Expat. This should only be of concern to those
subclassing XML::Parser.

=back

=item  setHandlers(TYPE, HANDLER [, TYPE, HANDLER [...]])

This method registers handlers for various parser events. It overrides any
previous handlers registered through the Style or Handler options or through
earlier calls to setHandlers. By providing a false or undefined value as
the handler, the existing handler can be unset.

This method returns a list of type, handler pairs corresponding to the
input. The handlers returned are the ones that were in effect prior to
the call.

See a description of the handler types in L<"HANDLERS">.

=item parse(SOURCE [, OPT => OPT_VALUE [...]])

The SOURCE parameter should either be a string containing the whole XML
document, or it should be an open IO::Handle. Constructor options to
XML::Parser::Expat given as keyword-value pairs may follow the SOURCE
parameter. These override, for this call, any options or attributes passed
through from the XML::Parser instance.

A die call is thrown if a parse error occurs. Otherwise it will return 1
or whatever is returned from the B<Final> handler, if one is installed.
In other words, what parse may return depends on the style.

=item parsestring

This is just an alias for parse for backwards compatibility.

=item parsefile(FILE [, OPT => OPT_VALUE [...]])

Open FILE for reading, then call parse with the open handle. The file
is closed no matter how parse returns. Returns what parse returns.

=item parse_start([ OPT => OPT_VALUE [...]])

Create and return a new instance of XML::Parser::ExpatNB. Constructor
options may be provided. If an init handler has been provided, it is
called before returning the ExpatNB object. Documents are parsed by
making incremental calls to the parse_more method of this object, which
takes a string. A single call to the parse_done method of this object,
which takes no arguments, indicates that the document is finished.

If there is a final handler installed, it is executed by the parse_done
method before returning and the parse_done method returns whatever is
returned by the final handler.

=back

=head1 HANDLERS

Expat is an event based parser. As the parser recognizes parts of the
document (say the start or end tag for an XML element), then any handlers
registered for that type of an event are called with suitable parameters.
All handlers receive an instance of XML::Parser::Expat as their first
argument. See L<XML::Parser::Expat/"METHODS"> for a discussion of the
methods that can be called on this object.

=head2 Init        (Expat)

This is called just before the parsing of the document starts.

=head2 Final        (Expat)

This is called just after parsing has finished, but only if no errors
occurred during the parse. Parse returns what this returns.

=head2 Start        (Expat, Element [, Attr, Val [,...]])

This event is generated when an XML start tag is recognized. Element is the
name of the XML element type that is opened with the start tag. The Attr &
Val pairs are generated for each attribute in the start tag.

=head2 End        (Expat, Element)

This event is generated when an XML end tag is recognized. Note that
an XML empty tag (<foo/>) generates both a start and an end event.

=head2 Char        (Expat, String)

This event is generated when non-markup is recognized. The non-markup
sequence of characters is in String. A single non-markup sequence of
characters may generate multiple calls to this handler. Whatever the
encoding of the string in the original document, this is given to the
handler in UTF-8.

=head2 Proc        (Expat, Target, Data)

This event is generated when a processing instruction is recognized.

=head2 Comment        (Expat, Data)

This event is generated when a comment is recognized.

=head2 CdataStart    (Expat)

This is called at the start of a CDATA section.

=head2 CdataEnd        (Expat)

This is called at the end of a CDATA section.

=head2 Default        (Expat, String)

This is called for any characters that don't have a registered handler.
This includes both characters that are part of markup for which no
events are generated (markup declarations) and characters that
could generate events, but for which no handler has been registered.

Whatever the encoding in the original document, the string is returned to
the handler in UTF-8.

=head2 Unparsed        (Expat, Entity, Base, Sysid, Pubid, Notation)

This is called for a declaration of an unparsed entity. Entity is the name
of the entity. Base is the base to be used for resolving a relative URI.
Sysid is the system id. Pubid is the public id. Notation is the notation
name. Base and Pubid may be undefined.

=head2 Notation        (Expat, Notation, Base, Sysid, Pubid)

This is called for a declaration of notation. Notation is the notation name.
Base is the base to be used for resolving a relative URI. Sysid is the system
id. Pubid is the public id. Base, Sysid, and Pubid may all be undefined.

=head2 ExternEnt    (Expat, Base, Sysid, Pubid)

This is called when an external entity is referenced. Base is the base to be
used for resolving a relative URI. Sysid is the system id. Pubid is the public
id. Base, and Pubid may be undefined.

This handler should either return a string, which represents the contents of
the external entity, or return an open filehandle that can be read to obtain
the contents of the external entity, or return undef, which indicates the
external entity couldn't be found and will generate a parse error.

If an open filehandle is returned, it must be returned as either a glob
(*FOO) or as a reference to a glob (e.g. an instance of IO::Handle).

A default handler is installed for this event. The default handler is
XML::Parser::lwp_ext_ent_handler unless the NoLWP option was provided with
a true value, otherwise XML::Parser::file_ext_ent_handler is the default
handler for external entities. Even without the NoLWP option, if the
URI or LWP modules are missing, the file based handler ends up being used
after giving a warning on the first external entity reference.

The LWP external entity handler will use proxies defined in the environment
(http_proxy, ftp_proxy, etc.).

Please note that the LWP external entity handler reads the entire
entity into a string and returns it, where as the file handler opens a
filehandle.

Also note that the file external entity handler will likely choke on
absolute URIs or file names that don't fit the conventions of the local
operating system.

The expat base method can be used to set a basename for
relative pathnames. If no basename is given, or if the basename is itself
a relative name, then it is relative to the current working directory.

=head2 ExternEntFin    (Expat)

This is called after parsing an external entity. It's not called unless
an ExternEnt handler is also set. There is a default handler installed
that pairs with the default ExternEnt handler.

If you're going to install your own ExternEnt handler, then you should
set (or unset) this handler too.

=head2 Entity        (Expat, Name, Val, Sysid, Pubid, Ndata, IsParam)

This is called when an entity is declared. For internal entities, the Val
parameter will contain the value and the remaining three parameters will be
undefined. For external entities, the Val parameter will be undefined, the
Sysid parameter will have the system id, the Pubid parameter will have the
public id if it was provided (it will be undefined otherwise), the Ndata
parameter will contain the notation for unparsed entities. If this is a
parameter entity declaration, then the IsParam parameter is true.

Note that this handler and the Unparsed handler above overlap. If both are
set, then this handler will not be called for unparsed entities.

=head2 Element        (Expat, Name, Model)

The element handler is called when an element declaration is found. Name
is the element name, and Model is the content model as an XML::Parser::Content
object. See L<XML::Parser::Expat/"XML::Parser::ContentModel Methods">
for methods available for this class.

=head2 Attlist        (Expat, Elname, Attname, Type, Default, Fixed)

This handler is called for each attribute in an ATTLIST declaration.
So an ATTLIST declaration that has multiple attributes will generate multiple
calls to this handler. The Elname parameter is the name of the element with
which the attribute is being associated. The Attname parameter is the name
of the attribute. Type is the attribute type, given as a string. Default is
the default value, which will either be "#REQUIRED", "#IMPLIED" or a quoted
string (i.e. the returned string will begin and end with a quote character).
If Fixed is true, then this is a fixed attribute.

=head2 Doctype        (Expat, Name, Sysid, Pubid, Internal)

This handler is called for DOCTYPE declarations. Name is the document type
name. Sysid is the system id of the document type, if it was provided,
otherwise it's undefined. Pubid is the public id of the document type,
which will be undefined if no public id was given. Internal is the internal
subset, given as a string. If there was no internal subset, it will be
undefined. Internal will contain all whitespace, comments, processing
instructions, and declarations seen in the internal subset. The declarations
will be there whether or not they have been processed by another handler
(except for unparsed entities processed by the Unparsed handler). However,
comments and processing instructions will not appear if they've been processed
by their respective handlers.

=head2 * DoctypeFin        (Parser)

This handler is called after parsing of the DOCTYPE declaration has finished,
including any internal or external DTD declarations.

=head2 XMLDecl        (Expat, Version, Encoding, Standalone)

This handler is called for xml declarations. Version is a string containg
the version. Encoding is either undefined or contains an encoding string.
Standalone will be either true, false, or undefined if the standalone attribute
is yes, no, or not made respectively.

=head1 STYLES

=head2 Debug

This just prints out the document in outline form. Nothing special is
returned by parse.

=head2 Subs

Each time an element starts, a sub by that name in the package specified
by the Pkg option is called with the same parameters that the Start
handler gets called with.

Each time an element ends, a sub with that name appended with an underscore
("_"), is called with the same parameters that the End handler gets called
with.

Nothing special is returned by parse.

=head2 Tree

Parse will return a parse tree for the document. Each node in the tree
takes the form of a tag, content pair. Text nodes are represented with
a pseudo-tag of "0" and the string that is their content. For elements,
the content is an array reference. The first item in the array is a
(possibly empty) hash reference containing attributes. The remainder of
the array is a sequence of tag-content pairs representing the content
of the element.

So for example the result of parsing:

  <foo><head id="a">Hello <em>there</em></head><bar>Howdy<ref/></bar>do</foo>

would be:
             Tag   Content
  ==================================================================
  [foo, [{}, head, [{id => "a"}, 0, "Hello ",  em, [{}, 0, "there"]],
          bar, [         {}, 0, "Howdy",  ref, [{}]],
            0, "do"
    ]
  ]

The root document "foo", has 3 children: a "head" element, a "bar"
element and the text "do". After the empty attribute hash, these are
represented in it's contents by 3 tag-content pairs.

=head2 Objects

This is similar to the Tree style, except that a hash object is created for
each element. The corresponding object will be in the class whose name
is created by appending "::" and the element name to the package set with
the Pkg option. Non-markup text will be in the ::Characters class. The
contents of the corresponding object will be in an anonymous array that
is the value of the Kids property for that object.

=head2 Stream

This style also uses the Pkg package. If none of the subs that this
style looks for is there, then the effect of parsing with this style is
to print a canonical copy of the document without comments or declarations.
All the subs receive as their 1st parameter the Expat instance for the
document they're parsing.

It looks for the following routines:

=over 4

=item * StartDocument

Called at the start of the parse .

=item * StartTag

Called for every start tag with a second parameter of the element type. The $_
variable will contain a copy of the tag and the %_ variable will contain
attribute values supplied for that element.

=item * EndTag

Called for every end tag with a second parameter of the element type. The $_
variable will contain a copy of the end tag.

=item * Text

Called just before start or end tags with accumulated non-markup text in
the $_ variable.

=item * PI

Called for processing instructions. The $_ variable will contain a copy of
the PI and the target and data are sent as 2nd and 3rd parameters
respectively.

=item * EndDocument

Called at conclusion of the parse.

=back

=head1 ENCODINGS

XML documents may be encoded in character sets other than Unicode as
long as they may be mapped into the Unicode character set. Expat has
further restrictions on encodings. Read the xmlparse.h header file in
the expat distribution to see details on these restrictions.

Expat has built-in encodings for: C<UTF-8>, C<ISO-8859-1>, C<UTF-16>, and
C<US-ASCII>. Encodings are set either through the XML declaration
encoding attribute or through the ProtocolEncoding option to XML::Parser
or XML::Parser::Expat.

For encodings other than the built-ins, expat calls the function
load_encoding in the Expat package with the encoding name. This function
looks for a file in the path list @XML::Parser::Expat::Encoding_Path, that
matches the lower-cased name with a '.enc' extension. The first one it
finds, it loads.

If you wish to build your own encoding maps, check out the XML::Encoding
module from CPAN.

=head1 AUTHORS

Larry Wall <F<[e-mail]>> wrote version 1.0.

Clark Cooper <F<[e-mail]>> picked up support, changed the API
for this version (2.x), provided documentation,
and added some standard package features.

=cut