Positive negative and mixed digital filtering over an arbitrary variable length bit string of a datastream by evaluating bits, bytes or any other desired granularity in accordance with a mask, a filter and a not match byte. Results are accumulated over a plurality of data blocks by ANDing of compare result values similarly representing match and not match results identically depending on the not match bit except where negayive logic has been applied over an entire data block. The preferred form of the digital filter is particularly adapted to be MPEG-2 compliant.