An apparatus for detecting presence of a selected complex string in a data stream has a primary processor, detecting coherent strings of the selected complex string; and a secondary processor, pipelined to the primary processor, and detecting ambiguous strings interleaving said coherent strings. The primary processor is configured to determine a number of coherent strings in the selected complex string, each coherent string preceded by a prefix of indefinite characters, with a last coherent string succeeded by a suffix of indefinite characters; and to locate sequential portions of the data stream that are identical to successive coherent strings of the selected complex string. The secondary processor is configured to ascertain congruence of portions of the data stream preceding and interleaving the sequential portions to prefixes of the coherent strings; and to ascertain congruence of the suffix to a corresponding portion of said data stream following the sequential portions.