1. An apparatus for detecting presence of a target complex string in a data stream, said apparatus comprising: a first processor executing instructions to locate coherent strings of said target complex string in said data stream, said target complex string comprising a number of ambiguous strings interleaved by coherent strings, each character in said coherent strings being uniquely defined in an alphabet, each character in said ambiguous strings belonging to one of predefined classes of characters; and a memory device storing processor executable instructions which cause a second processor to: access a table of said predefined classes of characters, each class corresponding to a respective subset of characters, to ascertain equality of classes of likewise positioned characters of portions of said data stream which interleave said coherent strings and respective ambiguous strings of said target complex string; and perform corrective actions subject to locating said coherent strings and ascertaining said equality of classes.