HTML::WikiConverter::Dialects(3) How to add a dialect


# In your dialect module:
package HTML::WikiConverter::MySlimWiki;
use base 'HTML::WikiConverter';
sub rules { {
b => { start => '**', end => '**' },
i => { start => '//', end => '//' },
strong => { alias => 'b' },
em => { alias => 'i' },
hr => { replace => "\n----\n" }
} }
# In a nearby piece of code:
package main;
use Test::More tests => 5;
my $wc = new HTML::WikiConverter(
dialect => 'MySlimWiki'
is( $wc->html2wiki( '<b>text</b>' ), '**text**', b );
is( $wc->html2wiki( '<i>text</i>' ), '//text//', i );
is( $wc->html2wiki( '<strong>text</strong>' ), '**text**', 'strong' );
is( $wc->html2wiki( '<em>text</em>' ), '//text//', 'em' );
is( $wc->html2wiki( '<hr/>' ), '----', 'hr' );


HTML::WikiConverter (or H::WC, for short) 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 to create your own dialect to be plugged into HTML::WikiConverter.


Each dialect has a separate dialect module containing rules for converting HTML into wiki markup specific for that dialect. Currently, all dialect modules are in the "HTML::WikiConverter::" package space and subclass HTML::WikiConverter. For example, the MediaWiki dialect module is HTML::WikiConverter::MediaWiki, while PhpWiki's is HTML::WikiConverter::PhpWiki. However, dialect modules need not be in the "HTML::WikiConverter::" package space; you may just as easily use "package MyWikiDialect;" and H::WC will Do The Right Thing.

From now on, I'll be using the terms ``dialect'' and ``dialect module'' interchangeably.


To interface with H::WC, dialects need to subclass it. This is done like so at the start of the dialect module:

  package HTML::WikiConverter::MySlimWiki;
  use base 'HTML::WikiConverter';

Conversion rules

Dialects guide H::WC's conversion process with a set of rules that define how HTML elements are turned into their wiki counterparts. Each rule corresponds to an HTML tag and there may be any number of rules. Rules are specified in your dialect's "rules()" method, which returns a reference to a hash of rules. Each entry in the hash maps a tag name to a set of subrules, as in:

    $tag => \%subrules

where $tag is the name of the HTML tag (e.g., "b", "em", etc.) and %subrules contains subrules that specify how that tag will be converted when it is encountered in the HTML input.


The following subrules are recognized:


A simple example

The following rules could be used for a dialect that uses "*asterisks*" for bold and "_underscores_" for italic text:

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


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

  strong => { alias => 'b' },
  em => { alias => 'i' },

(The "alias" subrule cannot be used with any other subrule.)


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

  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 an element be contained on a single line of text, or that it cannot contain any newlines, etc. These options can be specified using the "line_format" subrule, which can be assigned the value "single", "multi", or "blocks".

If the element must be contained on a single line, then the "line_format" subrule should be "single". If the element can span multiple lines, but there can be no blank lines contained within, then use "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" subrule 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. This is specified with the "line_prefix" subrule. For example, preformatted text in MediaWiki is 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" subrule. For example, in PhpWiki, three percent signs, "%%%", represents a line break, "<br>", hence:

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

(The "replace" subrule cannot be used with any other subrule.)

Preserving HTML tags

Some dialects allow a subset of HTML in their markup. While H::WC ignores unhandled HTML tags by default (i.e., if H::WC encounters a tag that does not exist in a dialect's rule specification, then the contents of the tag is simply passed through to the wiki markup), you may specify that some be preserved using the "preserve" subrule. 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" subrule:

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

(The "attributes" subrule can only be used if the "preserve" subrule is also present.)

Some HTML elements have no content (e.g., line breaks, images) and the wiki dialect might require them to be preserved in a more XHTML-friendly way. To indicate that a preserved tag should have no content, use the "empty" subrule. This will cause the element to be replaced with "<tag />" and 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, for example, "<br clear='both'>" into "<br clear='both' />". Without specifying the "empty" subrule, this would be converted into the (probably undesirable) "<br clear='both'></br>".

(The "empty" subrule can only be used if the "preserve" subrule is also present.)

Rules that depend on attribute values

In some circumstances, you might want your dialect's conversion rules to depend on the value of one or more attributes. This can be achieved by producing rules in a conditional manner within "rules()". For example:

  sub rules {
    my $self = shift;
    my %rules = (
      em => { start => "''", end => "''" },
      strong => { start => "'''", end => "'''" },
    $rules{i} = { preserve => 1 } if $self->preserve_italic;
    $rules{b} = { preserve => 1 } if $self->preserve_bold;
    return \%rules;

Dynamic subrules

Instead of simple strings, you may use coderefs as values for the "start", "end", "replace", and "line_prefix" subrules. If you do, the code will be called when the subrule is applied, and will be passed three arguments: the current H::WC object, the current HTML::Element node being operated on, and a reference to the hash containing the dialect's subrules associated with 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' },

It then defines "_li_start()":

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

This prefixes every unordered list item with "*" and every ordered list item with "1.", which MoinMoin requires. It also puts each list item on its own line and places a space between the prefix and the content of the list item.

Subrule validation

Certain subrule combinations are not allowed. Hopefully it's intuitive why this is, but in case it's not, prohibited combinations have been mentioned above parenthetically. For example, the "replace" and "alias" subrules cannot be combined with any other subrules, and "attributes" can only be specified alongside "preserve". Invalid subrule combinations will trigger a fatal error when the H::WC object is instantiated.

Dialect attributes

H::WC's constructor accepts a number of attributes that help determine how conversion takes place. Dialects can alter these attributes or add their own by defining an "attributes()" method, which returns a reference to a hash of attributes. Each entry in the hash maps the attribute's name to an attribute specification, as in:

  $attr => \%spec

where $attr is the name of the attribute and %spec is a Params::Validate specification for the attribute.

For example, to add a boolean attribute called "camel_case" which is disabled by default:

  sub attributes {
    camel_case => { default => 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;

You may override the default H::WC attributes using this mechanism. For example, while H::WC considers the "base_uri" attribute optional, it is required for the PbWiki dialect. PbWiki can override this default-optional behavior by saying:

  sub attributes {
    base_uri => { optional => 0 }


The first step H::WC takes 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 can define a "preprocess_node" method that will be called on each node of the tree (traversal is done in pre-order). The method receives two arguments, the H::WC object, and the current HTML::Element node being traversed. It may modify the node or decide to ignore it; its return value is discarded.

Built-in preprocessors

Because they are commonly needed, H::WC automatically carries out two preprocessing steps, 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 a "</td>" and "<td>") is discarded.

H::WC also provides additional preprocessing steps that may be explicitly enabled by dialect modules.

Removes 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 is complete, it is sometimes useful to postprocess the resulting wiki markup. Postprocessing can be used to clean up whitespace, fix subtle bugs introduced in the markup during conversion, etc.

Dialects that want to postprocess the wiki markup should define a "postprocess_output" method that will be called just before the "html2wiki" method returns to the client. The method will be passed two arguments, the H::WC object and a reference to the wiki markup. The method may modify the wiki markup that the reference points to; its return value is discarded.

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

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

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

Dialect utility methods

H::WC defines a set of utility methods that dialect modules may find useful.


  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". See ``ATTRIBUTES'' in HTML::WikiConverter for details on how the "wiki_uri" attribute is interpreted.


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

Returns true if $str is in CamelCase, false otherwise. CamelCase-ness is determined using the same rules that 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 contains 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"'.


  my $value = $wc->_attr( $name );

Returns the value of the named attribute. This is rarely needed since you can access attribute values by treating the attribute name as a method (i.e., "$wc->$name"). This low-level method of accessing attributes is provided for when you need to override an attribute's accessor/mutator method, as in:

  sub attributes { {
    my_attr => { default => 1 },
  } }
  sub my_attr {
    my( $wc, $name, $new_value ) = @_;
    # do something special
    return $wc->_attr( $name => $new_value );


David J. Iberri <[email protected]>


Copyright 2006 David J. Iberri, all rights reserved.

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