Detecting Patterns Can Be Hard: Circuit Lower Bounds for the Pattern Matching Problem

Detecting patterns in strings and images is a fundamental and widely studied problem. Motivated by the proliferation of specialized circuits in pattern recognition tasks, we study the circuit complexity of pattern matching under two popular choices of gates: De Morgan and threshold gates. For strings of length n and patterns of length log n k ≤ n− o(n), we prove super polynomial lower bounds for De Morgan circuits of depth 2, and nearly linear lower bounds for depth 2 threshold circuits. For unbounded depth and k ≥ 2, we prove a linear lower bound for (unbounded fan-in) De Morgan circuits. For certain values of k, we prove a Ω( √ n/ log n) lower bound for general threshold circuits and a nearly linear lower bound for depth 2 threshold circuits. Our proof for threshold circuits builds on a curious connection between detecting patterns and evaluating Boolean functions when the truth table of the function is given explicitly. Finally, we provide upper bounds on the size of circuits that solve the pattern matching problem. ∗Yahoo Research. Email: [email protected] †Department of Electrical Engineering and Computer Science, UC Berkeley. Email: [email protected] ‡Department of Electrical Engineering and Computer Science, UC Berkeley. Email: [email protected]