About this tool
What it is, why it matters, and how to use it without common mistakes.
What is it?
A regex tester lets you write a regular expression, apply it to sample text, and see matches, groups, and failures instantly — usually in JavaScript flavor in the browser.
Why use it?
Regex is dense. Live feedback prevents shipping patterns that silently miss edge cases in emails, URLs, logs, and form validation.
How it works
- Enter the pattern and optional flags (g, i, m, s, u).
- Provide sample input that includes both positive and negative cases.
- Inspect matches and capture groups; tighten the pattern until false positives disappear.
- Copy the final expression into your language's regex API.
Examples
Simple email-shaped pattern (illustrative)
^[^\s@]+@[^\s@]+\.[^\s@]+$
Best practices
- Prefer explicit character classes over . when possible.
- Anchor (^ $) when you mean “whole string”, not substring.
- Document why the pattern exists next to the code.
Common mistakes
- Catastrophic backtracking on untrusted input.
- Assuming one flavor (PCRE vs JS vs Python) behaves identically.
- Using regex where a proper parser (HTML, JSON, CSV) is safer.
FAQ
- Which regex flavor does code.live use?
- Browser tools use the JavaScript RegExp engine unless a page states otherwise.