The fast_pattern keyword is a content modifier that sets the content within a rule to be used with the fast pattern matcher. Since the default behavior of fast pattern determination is to use the longest content in the rule, it is useful if a shorter content is more "unique" than the longer content, meaning the shorter content is less likely to be found in a packet than the longer content.
The fast pattern matcher is used to select only those rules that have a chance of matching by using a content in the rule for selection and only evaluating that rule if the content is found in the payload. Though this may seem to be overhead, it can significantly reduce the number of rules that need to be evaluated and thus increases performance. The better the content used for the fast pattern matcher, the less likely the rule will needlessly be evaluated.
As this keyword is a modifier to the previous content keyword, there must be a content rule option in the rule before fast_pattern is specified. The fast_pattern option may be specified only once per rule.
|Note: The fast_pattern modifier cannot be used with the following http content modifiers: http_cookie, http_raw_uri, http_raw_header, http_raw_cookie, http_method, http_stat_code, http_stat_msg.|
|Note: The fast_pattern modifier can be used with negated contents only if those contents are not modified with offset, depth, distance or within.|