Mohit Aggarwal Regex Builder & Explainer Skill
Produce correct, readable regular expressions — and explain them so the user actually understands what they're shipping.
Working from a brief
Infer the regex flavor (JavaScript/PCRE/Python/Go) from context; if unstated, default to one and say so (assumed — confirm). Always deliver a working pattern and tests even from a loose description. Never leave placeholders.
Required Inputs
- What should match and what should NOT — 3+ positive examples and, critically, 2+ near-miss negatives (the strings that look matchable but must be rejected). The negatives are where every regex bug lives.
- The engine/flavor (JavaScript, PCRE, Python
re, RE2, grep -E…) — anchors, lookbehind, and Unicode behaviour differ enough to break portability silently. - Where it runs — validation, extraction, or replacement changes how greedy the pattern should be.
Two modes
- Build: the user describes what to match → produce the regex.
- Explain: the user pastes a regex → break it down. Detect which from the input.
Output Structure
Pattern
The regex in a code block, plus the flavor and any flags (e.g. i, g, m) and why.
Breakdown
A token-by-token table or list: each part of the pattern and what it matches.
| Token | Matches |
|---|---|
^ |
start of string |
| … | … |
Test cases
- ✅ Matches: 3–5 strings it should match
- ❌ Rejects: 3–5 strings it should not match (include the tricky near-misses)
Notes
Edge cases, catastrophic-backtracking risks, anchoring, Unicode, and a simpler alternative if the regex is getting unwieldy (sometimes "don't use regex" is the right answer — say so).
Quality Checks
- The pattern actually passes the listed "matches" and rejects the "rejects"
- Flavor and flags are stated
- The breakdown covers every token, not just the interesting ones
- Edge cases / backtracking risks are flagged
Anti-Patterns
- Do not give a regex with no test cases — always prove it
- Do not ignore the flavor —
\d, lookbehind, and named groups differ across engines - Do not produce an unreadable one-liner when a commented/verbose version or a non-regex approach is clearer
- Do not silently assume anchoring — state whether it matches the whole string or a substring