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Regex Tester
Test and debug regular expressions against your text — with live matches.
LIVE MATCHING
Test a pattern locally
Regex testing happens in your browser and your text is never uploaded.
Regex tester guide
Regular expressions are the most powerful — and most error-prone — text-matching tool in programming. This tester lets you write a pattern, paste sample text, and see every match highlighted live, with the exact match count and a clear error message when the pattern is invalid.
Everything runs in your browser with standard JavaScript regex syntax, so what you test here behaves identically in your code.
The building blocks
Regex is a small language for describing text patterns. The essentials:
- Literals — "cat" matches the characters c-a-t.
- Character classes — [0-9] any digit, [a-z] lowercase letter, \d digit, \w word character, \s whitespace.
- Quantifiers — * zero or more, + one or more, ? optional, {2,4} between 2 and 4.
- Anchors — ^ start of string/line, $ end, \b word boundary.
- Groups — (abc) capture group, (?:abc) non-capturing, | alternation (this|that).
The classic email pattern \b\w+@\w+\.\w+\b is pre-filled — try it, then experiment.
Flags explained
Flags change how the pattern behaves:
- g (global) — find all matches instead of stopping at the first.
- i (ignore case) — match both cases.
- m (multiline) — ^ and $ match line boundaries, not just string boundaries.
- s (dotall) — the dot matches newlines too.
- u (unicode) — proper Unicode handling for \w, case folding, and code points.
Testing discipline
The fastest way to write a correct regex: test against both your target cases and your edge cases (empty input, uppercase, unexpected punctuation). The live highlighting makes both visible at once — if your match highlights something you did not intend, you will see it immediately.
A common trap: regex is greedy by default. "a.+z" matches as much as possible, so "abczdefz" matches the whole string, not the first "abcz". Add a ? ("a.+?z") for lazy matching, or be specific about what you want.
Frequently asked questions
What is a regular expression?
A pattern language for matching text — sequences, character classes, quantifiers, and anchors that describe what text should look like. Used everywhere from validation to search-and-replace.
Why is my regex not matching?
Check for: missing flags (g for all matches, i for case), greedy quantifiers matching more than expected, and unescaped special characters (use \ to escape . * + ? [ ] etc.).
What do the flags g, i, m, s, u do?
g = all matches, i = case-insensitive, m = multiline anchors, s = dot matches newlines, u = unicode mode. Toggle them in the tester and watch results change live.
Is this the same syntax as in my code?
Yes — JavaScript regex syntax (the ECMAScript standard), the same one Node.js and browsers use. Patterns tested here work verbatim in JS code.
Can regex parse HTML or JSON?
No — those are nested structures, not regular languages. Use a real parser. Regex is for flat text patterns.
Privacy note: regex testing happens in your browser. Your text is not submitted to NAB Tools.