# Custom Markup

## Introduction

Pm Wiki's markup translation engine is handled by a set of rules; each rule searches for a specific pattern in the markup text and replaces it with some replacement text. Internally, this is accomplished by using PHP's "preg_replace" function.

Rules are added to the translation engine via Pm Wiki's Markup() function, which looks like

Markup($name,$when, $pattern,$replace);

where $name is a unique name (a string) given to the rule, $when says when the rule should be applied relative to other rules, $pattern is the pattern to be searched for in the markup text, and $replace is what the pattern should be replaced with.

For example, here's the code that creates the rule for ''emphasized text'' (in scripts/stdmarkup.php):

Markup("em", "inline", "/''(.*?)''/", "<em>$1</em>"); Basically this statement says to create a rule called "em" to be performed with the other "inline" markups, and the rule replaces any text inside two pairs of single quotes with the same text ($1) surrounded by <em> and </em>.

The first two parameters to Markup() are used to specify the sequence in which rules should be applied. The first parameter provides a name for a rule -- "em" in the example above. We could've chosen other names such as "''", or even "twosinglequotes". In general Pm Wiki uses the markup itself to name the rule (i.e., Pm Wiki uses "''" instead of "em"), but to keep this example easier to read later on we'll use a mnemonic name for now.

The second parameter says that this rule is to be done along with the other "inline" markups. Pm Wiki divides the translation process into a number of phases:

_begin      start of translation
fulltext    translations to be performed on the full text
split       conversion of the full markup text into lines to be processed
directives  directive processing
inline      inline markups
block       block markups
style       style handling
_end        end of translation


Thus, specifying "inline" for the second parameter says that this rule should be applied when the other "inline" rules are being performed. If we want a rule to be performed with the directives -- i.e., before inline rules are processed, we would specify "directives" or "<inline" for the second parameter.

The third parameter is a Perl-compatible regular expression. Basically, it is a slash, a regular expression, another slash, and a set of optional modifiers.

The example uses the pattern string "/''(.*?)''/", which uses ''(.*)'' as the regular expression and no options. (The regular expression says "find two single quotes in succession, then as few arbitrary characters as are needed to make the match find something, then two additional single quotes in succession"; the parentheses "capture" a part of the wikitext for later use.)

The fourth parameter is the replacement text that should be inserted instead of the marked-up wikitext. You can use $1, $2, etc. to insert the text from the first, second etc. parenthesised part of the regular expression.

In the example, we have "<em>$1</em>", which is an <em>, the text matched by the first parentheses (i.e. by the .*? section of the pattern), and </em>. Here's a rule for @@monospaced@@ text: Markup("@@", "inline", "/@@(.*?)@@/", "<code>$1</code>");

and for a [:comment ...:] directive that is simply removed from the output:

Markup("comment", "directives", "/\$:comment .*?:\$/", '');

Okay, now how about the rule for '''strong emphasis'''? We have to be a bit careful here, because although this translation should be performed along with other inline markup, we also have to make sure that the rule for ''' is handled before the rule for '', because ''' also contains ''. The second parameter to Markup() can be used to specify the new rule's relationship to any other rule: