home / skills / jmerta / codex-skills / regex-builder

regex-builder skill

safe

This skill helps you build, test, and explain regular expressions across engines with runnable verification and concise rationale.

npx playbooks add skill jmerta/codex-skills --skill regex-builder

Review the files below or copy the command above to add this skill to your agents.

Files (1)

SKILL.md

1.8 KB

---
name: regex-builder
description: Build, test, and explain regular expressions against sample text or files using CLI tools (rg, python) and specific regex flavors. Use when asked to craft, debug, or validate regexes or search patterns.
---
# Regex builder

## Goal
Produce a correct, testable regex with rationale and runnable verification.

## Inputs to confirm (ask if missing)
- Target regex flavor/engine (PCRE, Python, JavaScript, .NET, RE2, etc.).
- Example matches and non-matches (at least 3 each when possible).
- Scope: single-line vs multi-line; file types/paths.
- Output needs: capture groups, named groups, replacement template.

## Workflow
1) Gather samples
   - Ask for sample text or identify files.
   - If files exist, locate with `rg --files` and preview with `rg -n`.
2) Choose tool
   - Prefer `rg` for quick match checks; use `rg -P` for PCRE features.
   - Use Python for detailed group inspection when needed.
3) Build incrementally
   - Start with a minimal literal anchor; expand piece by piece.
   - Add anchors/boundaries; handle whitespace and separators explicitly.
4) Validate
   - Show a command to test against samples.
   - Confirm no false positives by testing non-matches.
5) Deliver
   - Provide final regex, flags, and explanation.
   - Include a test command and expected match summary.
   - Note any tradeoffs/backtracking risks.

## CLI snippets (use as needed)
- Ripgrep check:
  - `rg -n "<regex>" path\\to\\file`
  - `rg -n -P "<regex>" path\\to\\file`
- Python quick test (adjust quoting for the shell):
  - `python -c "import re,sys;pat=re.compile(r'<regex>');data=sys.stdin.read();print([m.group(0) for m in pat.finditer(data)])" < path\\to\\file`

## Deliverables
- Regex + flags and any capture group map.
- Test command(s) and summary of matches/non-matches.
- Short explanation of the approach and known limitations.

Overview

This skill builds, tests, and explains regular expressions tailored to a specific regex flavor and sample data. It produces a correct, runnable pattern, verification commands for common CLI tools (rg, python), and a brief rationale. The output includes flags, capture group mapping, and known tradeoffs to help you safely deploy the regex.

How this skill works

I first gather the target regex engine and a set of positive and negative examples. I then construct the pattern incrementally, using literal anchors, boundaries, and explicit whitespace handling. Finally I validate the regex with suggested ripgrep or Python commands and report matches, false positives, and limitations.

When to use it

Crafting a new regex for search, validation, or extraction across files
Debugging an existing pattern that produces unexpected matches or misses
Adapting a regex to a specific engine (PCRE, Python, JavaScript, RE2, .NET)
Verifying capture groups and producing a replacement template
Checking file-scoped searches with ripgrep or doing detailed group inspection with Python

Best practices

Provide at least three positive and three negative examples before building
Specify the regex flavor and whether multi-line or single-line scope is required
Start with a minimal literal match and expand with groups and anchors
Prefer non-greedy quantifiers and explicit character classes to reduce backtracking
Validate against real files using rg -P for PCRE or python for detailed captures

Example use cases

Extract ISO dates from log files while avoiding false matches like 9999-99-99
Build a JavaScript safe email validation with named capture groups for local/domain
Convert a loose grep search into a precise PCRE for ripgrep across a codebase
Fix a Python regex that drops group 2 on some lines by showing group mapping and test
Create a replacement template that swaps first/last name using captured groups

FAQ

What information do you need to start?

Tell me the regex flavor, a few matching and non-matching examples, scope (single/multi-line), and any required capture or replacement format.

Which tool do you use for quick checks?

I prefer ripgrep (rg) for fast file checks and use rg -P for PCRE. Use Python when you need detailed group output or cross-engine verification.