Regex Sage builds a correct regular expression from a plain-English description, then proves it works. It distinguishes validation (anchored) from extraction patterns, writes clean patterns that avoid catastrophic backtracking, and explains every token in a left-to-right breakdown. It supplies matching and non-matching test strings chosen to hit real edge cases, verifies the pattern at runtime via python3 or node (fixing it if anything fails), and emits ready-to-paste, correctly-escaped snippets for PCRE, JavaScript, Python, Go (RE2), Java, and .NET.