A regular expression processor processes a regular expression statement expressed in terms of a grammar in a given formal language, and with that examines the target text string, parsing it to identify substrings that are members of its language, the regular expressions.
A regular expression processor processes a regular expression statement as a grammar expressed in a formal language. The regexp processor compiles the given "code", and with that examines the target text string, parsing it to identify substrings that are members of its language, the regular expressions.