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What is RE2?

RE2 is a regular expression engine designed to operate in time proportional to the size of the input, ensuring linear time complexity. RE2JS is a pure JavaScript port that achieves full architectural parity with the Go regexp implementation.

JavaScript's standard regular expression engine, RegExp, and many other widely used packages (Perl, Python, PCRE) use a backtracking implementation strategy. When a pattern presents alternatives like a|b, the engine tries to match subpattern a first; if that fails, it resets the input and tries subpattern b.

If such choices are deeply nested, this strategy requires an exponential number of passes over the input data, potentially exceeding the lifetime of the universe for large inputs. This creates a security risk known as Regular Expression Denial of Service (ReDoS) when accepting patterns from untrusted sources.

In contrast, RE2JS utilizes a combination of Deterministic Finite Automaton (DFA) and Nondeterministic Finite Automaton (NFA) strategies to explore all matches simultaneously in a single pass over the input data. This approach guarantees