Efficient Matching with Memoization for Regexes with Look-around and Atomic Grouping

Abstract

AbstractRegular expression (regex) matching is fundamental in many applications, especially in web services. However, matching by backtracking—preferred by most real-world implementations for its practical performance and backward compatibility—can suffer from so-called catastrophic backtracking, which makes the number of backtracking super-linear and leads to the well-known ReDoS vulnerability. Inspired by a recent algorithm by Davis et al. that runs in linear time for (non-extended) regexes, we study efficient backtracking matching for regexes with two common extensions, namely look-around and atomic grouping. We present linear-time backtracking matching algorithms for these extended regexes. Their efficiency relies on memoization, much like the one by Davis et al.; we also strive for smaller memoization tables by carefully trimming their range. Our experiments—we used some real-world regexes with the aforementioned extensions—confirm the performance advantage of our algorithms.