1. A system for processing regular expressions containing one or more subexpressions, comprising: means for storing information regarding one or more regular expressions, each containing one or more sub-expressions; means for comparing data to the stored information regarding expressions in only a single pass through the data; means for determining from the comparison, for any stored expression, the location within the data of the beginning and end of each sub-expression, and the end of the regular expression; and means for identifying from such determination the presence within the data of any one or more stored regular expressions containing one or more sub-expressions.