17. A tangible computer-readable storage medium containing instructions which, when executed by a processor, cause the processor to perform a string matching method, the method comprising: storing a plurality of signature strings in a database; assigning values to characters in the signature strings; calculating differences between the assigned values of the characters in the signature strings; identifying common features of the signature strings using the calculated differences; grouping the signature strings into signature groups according to the common features; receiving an input string into a memory; detecting predetermined features of the input string; comparing the predetermined features of the input string with the common features of one or more of the signature groups; and comparing the input string with one or more individual signature strings in one of the signature groups if the predetermined features of the input string match the common features of the one of the signature groups.
24. The tangible computer-readable storage medium of claim 17 , the method further comprising: purging the input string if the input string matches one of the signature strings.