The Rules Autotag module takes a light-weight approach for autotagging full-text content by matching taxonomy terms (and optional Synonyms). Rules Autotag depends on the Rules module. Functionality is provided on different layers:
The API function rules_autotag_extract($text, $vocabulary) extracts terms from a given text which match a given vocabulary.
The basic rules action Extract terms (rules_autotag_extract_action) takes a text and a vocabulary as input and returns all matched terms.
The wrapper rules action set Autotag to terms list (rules_autotag_tag) directly stores the extracted terms into a terms list/field.
Comparison to related modules:
Taxonomy Autotagger takes a more user-oriented approach with by providing a AHAH-driven Autotag button.
Auto Tagging sends node content through any number of third party services to determine what the content is about.
Provides a "boolean" field type with an additional state of "not set" in addition to the usual "true/false" or "on/off." A separate display string can be defined for each state.
The purpose of this old module is to archive/delete old nodes.
The original design of this was just for forums but seen how it could be used in other places.