XML::Checker::Parser(3) an XML::Parser that validates at parse time


use XML::Checker::Parser;
my %expat_options = (KeepCDATA => 1,
Handlers => [ Unparsed => \&my_Unparsed_handler ]);
my $parser = new XML::Checker::Parser (%expat_options);
eval {
local $XML::Checker::FAIL = \&my_fail;
$parser->parsefile ("fail.xml");
if ($@) {
# Either XML::Parser (expat) threw an exception or my_fail() died.
... your error handling code here ...
# Throws an exception (with die) when an error is encountered, this
# will stop the parsing process.
# Don't die if a warning or info message is encountered, just print a message.
sub my_fail {
my $code = shift;
die XML::Checker::error_string ($code, @_) if $code < 200;
XML::Checker::print_error ($code, @_);


XML::Checker::Parser extends XML::Parser

I hope the example in the SYNOPSIS says it all, just use XML::Checker::Parser as if it were an XML::Parser. See XML::Parser for the supported (expat) options.

You can also derive your parser from XML::Checker::Parser instead of from XML::Parser. All you should have to do is replace:

 package MyParser;
 @ISA = qw( XML::Parser );


 package MyParser;
 @ISA = qw( XML::Checker::Parser );

XML::Checker::Parser constructor

 $parser = new XML::Checker::Parser (SkipExternalDTD => 1, SkipInsignifWS => 1);

The constructor takes the same parameters as XML::Parser with the following additions:

By default, it will try to load external DTDs using LWP. You can disable this by setting SkipExternalDTD to 1. See External DTDs for details.
By default, it will treat insignificant whitespace as regular Char data. By setting SkipInsignifWS to 1, the user Char handler will not be called if insignificant whitespace is encountered. See ``INSIGNIFICANT_WHITESPACE'' in XML::Checker for details.
When calling parsefile() with a URL (instead of a filename) or when loading external DTDs, we use LWP to download the remote file. By default it will use a LWP::UserAgent that is created as follows:

 use LWP::UserAgent;
 $LWP_USER_AGENT = LWP::UserAgent->new;

Note that env_proxy reads proxy settings from your environment variables, which is what I need to do to get thru our firewall. If you want to use a different LWP::UserAgent, you can either set it globally with:

 XML::Checker::Parser::set_LWP_UserAgent ($my_agent);

or, you can specify it for a specific XML::Checker::Parser by passing it to the constructor:

 my $parser = new XML::Checker::Parser (LWP_UserAgent => $my_agent);

Currently, LWP is used when the filename (passed to parsefile) starts with one of the following URL schemes: http, https, ftp, wais, gopher, or file (followed by a colon.) If I missed one, please let me know.

The LWP modules are part of libwww-perl which is available at CPAN.

External DTDs

XML::Checker::Parser will try to load and parse external DTDs that are referenced in DOCTYPE definitions unless you set the SkipExternalDTD option to 1 (the default setting is 0.) See CAVEATS for details on what is not supported by XML::Checker::Parser.

XML::Parser (version 2.27 and up) does a much better job at reading external DTDs, because recently external DTD parsing was added to expat. Make sure you set the XML::Parser option ParseParamEnt to 1 and the XML::Checker::Parser option SkipExternalDTD to 1. (They can both be set in the XML::Checker::Parser constructor.)

When external DTDs are parsed by XML::Checker::Parser, they are located in the following order:

  • With the %URI_MAP, which can be set using map_uri. This hash maps external resource ids (like system ID's and public ID's) to full path URI's. It was meant to aid in resolving PUBLIC IDs found in DOCTYPE declarations after the PUBLIC keyword, e.g.


    However, you can also use this to force XML::Checker to read DTDs from a different URL than was specified (e.g. from the local file system for performance reasons.)

  • on the Internet, if their system identifier starts with a protocol (like http://...)
  • on the local disk, if their system identifier starts with a slash (absolute path)
  • in the SGML_SEARCH_PATH, if their system identifier is a relative file name. It will use @SGML_SEARCH_PATH if it was set with set_sgml_search_path(), or the colon-separated $ENV{SGML_SEARCH_PATH}, or (if that isn't set) the list (``.'', ``$ENV{'HOME'}/.sgml'', ``/usr/lib/sgml'', ``/usr/share/sgml''), which includes the current directory, so it should do the right thing in most cases.

Static methods related to External DTDs

set_sgml_search_path (dir1, dir2, ...)
External DTDs with relative file paths are looked up using the @SGML_SEARCH_PATH, which can be set with this method. If @SGML_SEARCH_PATH is never set, it will use the colon-separated $ENV{SGML_SEARCH_PATH} instead. If neither are set it uses the list: ``.'', ``$ENV{'HOME'}/.sgml'', ``/usr/lib/sgml'', ``/usr/share/sgml''.

set_sgml_search_path is a static method.

map_uri (pubid => uri, ...)
To define the location of PUBLIC ids, as found in DOCTYPE declarations after the PUBLIC keyword, e.g.


call this method, e.g.

  XML::Checker::Parser::map_uri (
        "-//W3C//DTD HTML 4.0//EN" => "file:/user/html.dtd");

See External DTDs for more info.

XML::Checker::Parser::map_uri is a static method.

Switching user handlers at parse time

You should be able to use setHandlers() just as in XML::Parser. (Using setHandlers has not been tested yet.)

Error handling

XML::Checker::Parser routes the fail handler through XML::Checker::Parser::fail_add_context() before calling your fail handler (i.e. the global fail handler: $XML::Checker::FAIL. See ``ERROR_HANDLING'' in XML::Checker.) It adds the (line, column, byte) information from XML::Parser to the error context (unless it was the end of the XML document.)

Supported XML::Parser handlers

Only the following XML::Parser handlers are currently routed through XML::Checker: Init, Final, Char, Start, End, Element, Attlist, Doctype, Unparsed, Notation.


When using XML::Checker::Parser to parse external DTDs (i.e. with SkipExternalDTD => 0), expect trouble when your external DTD contains parameter entities inside declarations or conditional sections. The external DTD should probably have the same encoding as the orignal XML document.


Enno Derksen is the original author.

Send bug reports, hints, tips, suggestions to T.J. Mather at <[email protected]>.