Manipulating User Input: The Filter System

Abstract

Adding content to a web site can be quite a chore when you have to format the information yourself. Conversely, making text input look good on a web site requires knowledge of HTML—knowledge most users don’t want to be bothered with. For those of us who are HTML-savvy, it’s still a pain to stop and insert tags into our post during the middle of a brainstorm or literary breakthrough. Paragraph tags, link tags, break tags... yuck. The good news is that Drupal uses prebuilt routines called filters to make data entry easy and efficient. Filters perform text manipulations such as making URLs clickable, converting line breaks to < p> and < br /> tags, and even stripping out malicious HTML. hook_filter() is the mechanism behind filter creation and manipulation of user-submitted data. Filters are almost always a single action such as “strip out all hyperlinks,” “add a random image to this post,” or even “translate this into pirate-speak” (see pirate.module at http://drupal.org/project/pirate).