# Regex Builder

> 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. Use when this capability is needed.

- Skill: `tomevault-io/regex-builder` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add tomevault-io/regex-builder`
- Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/regex-builder/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: tomevault-io (https://skillmd.com/u/tomevault-io)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/tomevault-io/regex-builder

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# 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.

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<!-- tomevault:4.0:skill_md:2026-04-11 -->