A single 40-character form value froze an API worker for minutes. A nested quantifier in a validation regex made Python's backtracking engine try exponentially many paths. We show how we proved it, rewrote the pattern and added RE2 and CI checks.
Writing a 200-character regex in one go is how parsing bugs are born. We build patterns the way we build code: named parts, explicit groups, alternation that stays contained, lookaheads for extra rules, then extract every field from log lines and key=value strings with tests that keep it honest.
A practical first pass at regular expressions with Python's re module: the tokens you use every day, quantifiers, anchors, and the difference between search, match and fullmatch, ending with one pattern that safely validates an upload filename.