What is the time and space complexity of Regular Expression Matching?

Time complexity: O(m * n). Space complexity: O(m * n).

Regular Expression Matching

Hard

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Solution Guide: Regular Expression Matching

Approach

Build DP over string/pattern prefixes. Handle normal char or dot with diagonal transition, and star with either zero-occurrence or consume-one-character transitions.

Complexity Analysis

Time: O(m * n)
Space: O(m * n)

Pattern

2D Dynamic Programming

Why It Works

Each pattern operator defines deterministic transitions from smaller prefix states, making full-match validity a DP reachability problem.

Updated Mar 2026