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HTML::WikiConverter::Dialects - How to add a dialect


  package HTML::WikiConverter::MySimpleWiki;
  use base 'HTML::WikiConverter';
  sub rules {
    b => { start => '*', end => '*' },
    i => { start => '/', end => '/' },
    strong => { alias => 'b' },
    em => { alias => 'i' },
    hr => { replace => "\n----\n" },
    br => { preserve => 1, empty => 1 }

  # In a nearby piece of code...
  my $wc = new HTML::WikiConverter(
    dialect => 'MySimpleWiki'

  # Outputs "*text*"
  print $wc->html2wiki( "<b>text</b>" );

  # Outputs "/text/"
  print $wc->html2wiki( "<em>text</em>" );


HTML::WikiConverter is an HTML to wiki converter. It can convert HTML source into a variety of wiki markups, called wiki "dialects". This manual describes how you might create your own dialect to be plugged into HTML::WikiConverter.


HTML::WikiConverter can convert HTML into markup for a variety of wiki dialects. The rules for converting HTML into a given dialect are specified in a dialect module registered in the HTML::WikiConverter:: namespace. For example, the rules for the MediaWiki dialect are provided in HTML::WikiConverter::MediaWiki, while PhpWiki's rules are specified in HTML::WikiConverter::PhpWiki.

This section is intended for dialect module authors.

Conversion rules

To interface with HTML::WikiConverter, dialect modules must define a single rules class method. It returns a reference to a hash of rules that specify how individual HTML elements are converted to wiki markup.

Supported rules

The following rules are recognized:






Simple rules method

For example, the following rules method could be used for a wiki dialect that uses *asterisks* for bold and _underscores_ for italic text:

  sub rules {
    return {
      b => { start => '*', end => '*' },
      i => { start => '_', end => '_' }


To add <strong> and <em> as aliases of <b> and <i>, use the alias rule:

  sub rules {
    return {
      b => { start => '*', end => '*' },
      strong => { alias => 'b' },

      i => { start => '_', end => '_' },
      em => { alias => 'i' }

Note that if you specify the alias rule, no other rules are allowed.


Many dialects separate paragraphs and other block-level elements with a blank line. To indicate this, use the block rule:

  p => { block => 1 }

To better support nested block elements, if a block elements are nested inside each other, blank lines are only added to the outermost element.

Line formatting

Many dialects require that the text of a paragraph be contained on a single line of text. Or perhaps that a paragraph cannot contain any newlines. These options can be specified using the line_format rule, which can be assigned the value "single", "multi", or "blocks".

If the element must be contained on a single line, then the line_format rule should be "single". If the element can span multiple lines, but there can be no blank lines contained within, then it should be "multi". If blank lines (which delimit blocks) are allowed, then use "blocks". For example, paragraphs are specified like so in the MediaWiki dialect:

  p => { block => 1, line_format => 'multi', trim => 'both' }

Trimming whitespace

The trim rule specifies whether leading or trailing whitespace (or both) should be stripped from the element. To strip leading whitespace only, use "leading"; for trailing whitespace, use "trailing"; for both, use the aptly named "both"; for neither (the default), use "none".

Line prefixes

Some elements require that each line be prefixed with a particular string. For example, preformatted text in MediaWiki s prefixed with a space:

  pre => { block => 1, line_prefix => ' ' }


In some cases, conversion from HTML to wiki markup is as simple as string replacement. To replace a tag and its contents with a particular string, use the replace rule. For example, in PhpWiki, three percent signs '%%%' represents a linebreak <br />, hence the rule:

  br => { replace => '%%%' }

(The replace rule cannot be used with any other rule.)

Preserving HTML tags

Some dialects allow a subset of HTML in their markup. HTML tags can be preserved using the preserve rule. For example, to allow <font> tag in wiki markup:

  font => { preserve => 1 }

Preserved tags may also specify a list of attributes that may also passthrough from HTML to wiki markup. This is done with the attributes option:

  font => { preserve => 1, attributes => [ qw/ font size / ] }

(The attributes rule must be used alongside the preserve rule.)

Some HTML elements have no content (e.g. line breaks, images), and should be preserved specially. To indicate that a preserved tag should have no content, use the empty rule. This will cause the element to be replaced with "<tag />", with no end tag. For example, MediaWiki handles line breaks like so:

  br => {
    preserve => 1,
    attributes => qw/ id class title style clear /,
    empty => 1

This will convert, e.g., "<br clear='both'>" into "<br clear='both' />". Without specifying the empty rule, this would be converted into the undesirable "<br clear='both'></br>".

The empty rule must be combined with the preserve rule.

Dynamic rules

Instead of simple strings, you may use coderefs as values for the start, end, replace, and line_prefix rules. If you do, the code will be called as a method on the current HTML::WikiConverter dialect object, and will be passed the current HTML::Element node and a hashref of the dialect's rules for processing elements of that type.

For example, MoinMoin handles lists like so:

  ul => { line_format => 'multi', block => 1, line_prefix => '  ' }
  li => { start => \&_li_start, trim => 'leading' }
  ol => { alias => 'ul' }

And then defines _li_start:

  sub _li_start {
    my( $self, $rules ) = @_;
    my $bullet = '';
    $bullet = '*'  if $node->parent->tag eq 'ul';
    $bullet = '1.' if $node->parent->tag eq 'ol';
    return "\n$bullet ";

This ensures that every unordered list item is prefixed with * and every ordered list item is prefixed with 1., required by the MoinMoin formatting rules. It also ensures that each list item is on a separate line and that there is a space between the prefix and the content of the list item.

Rule validation

Certain rule combinations are not allowed. For example, the replace and alias rules cannot be combined with any other rules, and attributes can only be specified alongside preserve. Invalid rule combinations will trigger an error when the HTML::WikiConverter object is instantiated.

Dialect attributes

The attributes that are recognized by the HTML::WikiConverter are given in the attributes method, which returns a hash of attribute names and their defaults. Dialects that wish to alter the set of recognized attributes must override this method. For example, to add a boolean attribute called camel_case with is disabled by default, a dialect would define an attributes method like so:

  sub attributes { (
    camel_case => 0
  ) }

Attributes defined liks this are given accessor and mutator methods via Perl's AUTOLOAD mechanism, so you can later say:

  my $ok = $wc->camel_case; # accessor
  $wc->camel_case(0); # mutator


The first step in converting HTML source to wiki markup is to parse the HTML into a syntax tree using HTML::TreeBuilder. It is often useful for dialects to preprocess the tree prior to converting it into wiki markup. Dialects that need to preprocess the tree define a preprocess_node method that will be called on each node of the tree (traversal is done in pre-order). As its only argument the method receives the current HTML::Element node being traversed. It may modify the node or decide to ignore it. The return value of the preprocess_node method is discarded.

Built-in preprocessors

Because they are commonly needed, two preprocessing steps are automatically carried out by HTML::WikiConverter, regardless of the dialect: 1) relative URIs in images and links are converted to absolute URIs (based upon the base_uri parameter), and 2) ignorable text (e.g. between </td> and <td>) is discarded.

HTML::WikiConverter also provides additional preprocessing steps that may be explicitly enabled by dialect modules.


Removes from the HTML input any anchor elements that do not contain an href attribute.


Removes table captions and reinserts them as paragraphs before the table.

Dialects may apply these optional preprocessing steps by calling them as methods on the dialect object inside preprocess_node. For example:

  sub preprocess_node {
    my( $self, $node ) = @_;


Once the work of converting HTML, it is sometimes useful to postprocess the resulting wiki markup. Postprocessing can be used to clean up whitespace, fix subtle bugs in the markup that can't otherwise be done in the original conversion, etc.

Dialects that want to postprocess the wiki markup should define a postprocess_output object method that will be called just before thehtml2wiki method returns to the client. The method will be passed a single argument, a reference to the wiki markup. It may modify the wiki markup that the reference points to. Its return value is discarded.

For example, to convert a series of line breaks to be replaced with a pair of newlines, a dialect might implement this:

  sub postprocess_output {
    my( $self, $outref ) = @_;
    $$outref =~ s/<br>\s*<br>/\n\n/g;

(This example assumes that HTML line breaks were replaced with <br> in the wiki markup.)

Dialect utility methods

HTML::WikiConverter defines a set of utility methods for use by dialect modules.

  my $wiki = $wc->get_elem_contents( $node );

Converts the contents of $node into wiki markup and returns the resulting wiki markup.

  my $title = $wc->get_wiki_page( $url );

Attempts to extract the title of a wiki page from the given URL, returning the title on success, undef on failure. If wiki_uri is empty, this method always return undef. Assumes that URLs to wiki pages are constructed using "<wiki-uri><page-name>".

  my $ok = $wc->is_camel_case( $str );

Returns true if $str is in CamelCase, false otherwise. CamelCase-ness is determined using the same rules as CGI::Kwiki's formatting module uses.

  my $attr_str = $wc->get_attr_str( $node, @attrs );

Returns a string containing the specified attributes in the given node. The returned string is suitable for insertion into an HTML tag. For example, if $node refers to the HTML

  <style id="ht" class="head" onclick="editPage()">Header</span>

and @attrs contains "id" and "class", then get_attr_str will return 'id="ht" class="head"'.


David J. Iberri <diberri@cpan.org>


Copyright (c) 2004-2005 David J. Iberri

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

See http://www.perl.com/perl/misc/Artistic.html