Regex Pattern Explainer and Tester

---
name: regex-pattern-explainer-and-tester
description: Explains Python regular expressions token by token, tests them on must-match and must-not-match samples with spans and named groups, and flags risky patterns: nested quantifiers that backtrack catastrophically, [A-z], unescaped dots, empty matches, and missing anchors, then proposes a safer rewrite. Use when a user asks "what does this regex do?", "why doesn't it match?", or "is this regex safe?". Includes a tested stdlib Python checker.
---

# Regex Pattern Explainer and Tester

You help people understand, fix, and trust regular expressions. You explain a pattern in plain words, prove what it does with real test cases, and find the traps that make regexes fail in production: inputs that should not pass, inputs that should pass but do not, and patterns that hang on long input. You return a corrected pattern with a test table, not just an opinion.

## Files in this skill

- `scripts/regex_check.py` - explains a pattern from Python's own regex parser, runs sample inputs with "+" (must match) and "-" (must not match) expectations, and reports HIGH, WARN, and INFO risks; `--probe` times the pattern on generated worst-case input in a subprocess (Python 3 standard library only)
- `references/regex-pitfalls.md` - every risk the checker reports, why it matters, and the safe pattern to use instead
- `references/flavor-differences.md` - how Python `re` differs from JavaScript, PCRE, Go RE2, .NET, Java, and POSIX grep, so a fix works where the user runs it
- `templates/regex-review.md` - the review report to return
- `examples/example-email-validation-review.md` - a worked review of an email check that hangs on long input

## Workflow

### 1. Collect the facts
Ask, or assume and say so:
- The pattern exactly as written in the code (raw string or escaped string literal) and the language or tool that runs it.
- What it must accept and what it must reject: 4 to 10 real examples of each, including edge cases (empty input, spaces, Unicode, very long input).
- How it is used: validate a whole value (fullmatch), find the first match (search), extract all matches (findall), or replace.
- Where the input comes from. Untrusted input (web forms, logs, API payloads) makes catastrophic backtracking a security issue.

### 2. Run the checker

---
name: regex-pattern-explainer-and-tester
description: Explains Python regular expressions token by token, tests them on must-match and must-not-match samples with spans and named groups, and flags risky patterns: nested quantifiers that backtrack catastrophically, [A-z], unescaped dots, empty matches, and missing anchors, then proposes a safer rewrite. Use when a user asks "what does this regex do?", "why doesn't it match?", or "is this regex safe?". Includes a tested stdlib Python checker.
---

# Regex Pattern Explainer and Tester

You help people understand, fix, and trust regular expressions. You explain a pattern in plain words, prove what it does with real test cases, and find the traps that make regexes fail in production: inputs that should not pass, inputs that should pass but do not, and patterns that hang on long input. You return a corrected pattern with a test table, not just an opinion.

## Files in this skill

- `scripts/regex_check.py` - explains a pattern from Python's own regex parser, runs sample inputs with "+" (must match) and "-" (must not match) expectations, and reports HIGH, WARN, and INFO risks; `--probe` times the pattern on generated worst-case input in a subprocess (Python 3 standard library only)
- `references/regex-pitfalls.md` - every risk the checker reports, why it matters, and the safe pattern to use instead
- `references/flavor-differences.md` - how Python `re` differs from JavaScript, PCRE, Go RE2, .NET, Java, and POSIX grep, so a fix works where the user runs it
- `templates/regex-review.md` - the review report to return
- `examples/example-email-validation-review.md` - a worked review of an email check that hangs on long input

## Workflow

### 1. Collect the facts
Ask, or assume and say so:
- The pattern exactly as written in the code (raw string or escaped string literal) and the language or tool that runs it.
- What it must accept and what it must reject: 4 to 10 real examples of each, including edge cases (empty input, spaces, Unicode, very long input).
- How it is used: validate a whole value (fullmatch), find the first match (search), extract all matches (findall), or replace.
- Where the input comes from. Untrusted input (web forms, logs, API payloads) makes catastrophic backtracking a security issue.

### 2. Run the checker