Regex Tester — Test Regular Expressions Online
Test regular expressions with live match highlighting, capture groups, and flag toggles — debug your regex instantly. Free, client-side regex tester.
🔒 100% Client-Side Processing
Your data is processed entirely in your browser and never transmitted to any server.
//g
Common Use Cases
Debugging validation rules
Test the email, phone, or password patterns from your form validation against real user input before shipping — see exactly which strings match and which capture groups fire.
Building log filters
Prototype the pattern for a grep command, Splunk query, or log-shipping filter against pasted log lines, then copy the working regex into your pipeline.
Learning regex safely
Experiment with lookaheads, flags, and quantifiers with instant visual feedback — far faster than re-running a script for each attempt.
Frequently Asked Questions
- What regex flavor does this tester use?
- JavaScript (ECMAScript) regular expressions — the same engine as Node.js and every browser. Most PCRE syntax carries over, but PCRE-only features like possessive quantifiers, atomic groups, and recursion are not supported. Lookahead, lookbehind, named groups, and Unicode escapes all work.
- What do the g, i, m, and s flags do?
- g (global) finds every match instead of stopping at the first; i makes matching case-insensitive; m makes ^ and $ anchor at each line instead of the whole string; s makes the dot (.) match newline characters too.
- Why does my pattern hang or run slowly on long input?
- Usually catastrophic backtracking — nested quantifiers like (a+)+ or ambiguous alternations force the engine to try exponentially many paths. Rewrite so alternatives cannot match the same text, or anchor the pattern. This tester caps output at 1,000 matches to stay responsive.
- Does this work for testing Python, Java, or PCRE regex, not just JavaScript?
- It runs JavaScript's regex engine, so patterns match JS semantics exactly — which is very close to Python's re module and Java's Pattern class for everyday patterns (anchors, character classes, quantifiers, groups). Where flavors genuinely diverge — Python's (?P<name>...) named-group syntax, Java's possessive quantifiers, PCRE recursion — a pattern that passes here can still need small syntax adjustments in that language's own engine.
- What is the difference between a regex tester and a regex generator?
- A tester (this tool) runs a regex pattern you already have against sample text so you can see exactly what it matches and debug it. A generator instead tries to build a pattern for you from a description or example input — a different job this tool does not do; you bring the pattern, and this tool tells you what it actually matches.
- Is my data private when I use this tool?
- Yes. This tool runs entirely in your browser using client-side JavaScript — nothing you type is transmitted to, logged by, or stored on any server. You can safely process confidential text, tokens, or code.