Guide: how to test a regular expression in JavaScript #361

Context

Part of the SEO growth initiative (#18): Phase 2: Coverage engine (guides and internal linking).
Searches like "regex tester javascript" and "javascript regex named groups" map directly to the Regex Tester.

What to do

• Write a genuinely useful, original guide (examples, a table, or screenshots) that answers the query in the first two sentences.
• Use clear H2/H3 that match sub-questions, short paragraphs, and lists.
• Link to the Regex Tester (/regex-tester) and to related guides.
• Topics worth covering: the six flags the tester offers (g, i, m, s, u, y), numbered and named capture groups, $1, $<name> and $& in replacements, why a pattern like a* gives empty matches and how the tester steps past them, escaping special characters, and catastrophic backtracking with an example like (a+)+$.
• Check every statement about the tool against its code (app/(tools)/regex-tester/ and lib/regex-utils.ts) and every technical fact against the ECMAScript spec or MDN. Guides are reviewed line by line for accuracy.

Where

A new entry in GUIDES (lib/guides.ts) and a route under app/guides/<slug>/ (page.tsx and opengraph-image.tsx), set up the same way as the existing guides in app/guides/format-and-validate-json/ and app/guides/decode-jwt/. The "Add a guide" section of the README lists the steps.

Acceptance criteria

• Guide published, answers the query up top, and links to the Regex Tester
• Everything it says about the tool matches what the tool does today (for example the 1,000 match limit and the 100,000 character cap), and publishedAt is the day the guide goes live
• npm run lint && npm run test && npm run build passes

Notes

Want to work on this? Comment below and I'll assign it to you. Please take one issue at a time, branch off develop, and open your PR against develop with Closes #361 in the description.