Moving regexes between runtimes is risky because “looks similar” is not “behaves the same.” This playbook helps you migrate safely.

Step 1: Inventory Features

List everything used: named groups, lookbehind, unicode classes, atomic groups, backtracking control verbs, replacement syntax, and flags.

Step 2: Build a Shared Fixture Suite

Create a single fixture file of valid/invalid samples and run it in both old and new environments.

Step 3: Rewrite Incompatibilities

When features are missing (for example PCRE recursion in JavaScript), replace with parser logic or multi-pass checks.

Step 4: Verify Replacement Semantics

Capture and replacement syntax differ: JavaScript uses $1, Python often uses \1. Validate transformed output, not just match booleans.

Step 5: Observe Production Metrics

Track error rates and latency after rollout. Regex migrations can pass tests yet still shift performance under real traffic shapes.