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.