# `StringParser_BBCode` class documentation

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## 5. Filters

### 5.1 Filter types

In addition to the parser functions the class also provides so-called filters. Filters are functions that have to behave the same as parser functions. They have to accept a text as the only parameter and return a string. The class knows of two different filter types:

Prefilters
  These are filters that are called before the parsing process. The complete text that is to be parsed is passed through every filter and is processed not until then.
Postfilters
  These are filters that are called after the tree has been converted to a string again and just before the `parse` method returns the text.

### 5.2 Defining filters

Filters are defined similar to parser functions:

`$bbcode->addFilter (STRINGPARSER_FILTER_PRE, 'my_filter_function');
$bbcode->addFilter (STRINGPARSER_FILTER_POST, 'my_second_filter_function');`

The constants `STRINGPARSER_FILTER_PRE` and `STRINGPARSER_FILTER_POST` are important here. `STRINGPARSER_FILTER_PRE` indicates that the filter is a prefilter, `STRINGPARSER_FILTER_POST` that it is a postfilter.

Exactly as with parser functions filters may also be class methods:

`$bbcode->addFilter (STRINGPARSER_FILTER_PRE, array (&my_object, 'my_method'));`

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