DocomintRegex Tester: Test Regular Expressions Online
Regular expressions are one of those tools that are genuinely powerful and genuinely easy to get subtly wrong. You write a pattern, it looks right, and then it either fails to match the one input you actually cared about or matches something you never intended. A regex tester closes that gap — you see matches highlighted against real sample text immediately, instead of finding out in production.
Why "looks right" isn't good enough
A pattern like ^[^\s@]+@[^\s@]+\.[^\s@]+$ reads as a reasonable email matcher — until you test it against user+tag@sub.example.co.ukor a string with a trailing newline, and it behaves differently than expected. Regex bugs are almost always edge cases: greedy vs. lazy quantifiers, anchors that don't account for multiline input, character classes that are one character off. None of that is visible by reading the pattern alone — you have to run it against real strings.
What a live tester actually gives you
- Immediate feedback — edit the pattern, see matches update, without recompiling anything.
- Multiple test cases at once — paste several lines and see which ones match and which don't, side by side.
- Visual highlighting — matched substrings are shown inline, so you can spot exactly what a group captured.
How to test a regex pattern
- Open the Regex Tester.
- Enter your pattern and any flags (case-insensitive, global, multiline).
- Paste sample text — ideally including the edge cases you're worried about, not just the happy path.
- Matches highlight live as you adjust the pattern.
Testing happens entirely in your browser, which matters if your sample text includes anything you'd rather not paste into a random third-party tool — log lines with user data, internal URLs, or pieces of a config file.
A practical way to build a regex
Rather than writing the whole pattern at once, build it incrementally: start with the simplest version that matches your best-case input, then add real edge cases to your test text one at a time and adjust the pattern until each one passes without breaking the ones before it. That's far faster than trying to reason through quantifier precedence in your head.
Related developer tools
Once you've confirmed a pattern matches correctly, you'll often need to apply it somewhere that touches encoded text — checking a URL parameter, for instance, is easier once it's decoded with URL Encoder/Decoderfirst. And if the text you're testing against came out of a JSON payload, run it through the JSON Formatter to pull out the field cleanly.
Frequently asked questions
Which regex flavor does this test against?
The JavaScript regex engine — the same one your pattern will run against if you're using it in a browser or Node.js application.
Is my sample text uploaded anywhere?
No — matching runs entirely in your browser, which matters if your test text includes real log lines or user data.
Can I test a pattern against multiple strings at once?
Yes — paste multiple lines of sample text and see which ones match and which don't, all at once.
Does it show capture groups, not just the full match?
Yes — matched substrings and their groups are highlighted inline so you can see exactly what each part of the pattern captured.
Why does my pattern behave differently here than in my code?
Usually a flags mismatch — check whether your code applies the global, multiline, or case-insensitive flag differently than what you've set here.