Validate JSON and pinpoint errors by line and column — including duplicate keys that most parsers ignore.
Regex Tester
Test JavaScript regular expressions with live highlighting, capture groups and replace.
Runs in your browser — nothing you enter is sent to our servers
How it works
Your pattern and flags are compiled with JavaScript’s RegExp. Matching runs in a background Web Worker with a 1.5-second limit, so a runaway pattern can’t lock up the page. Matches are highlighted in the test string, and the table lists each match’s position with its numbered and named capture groups. The replace box uses String.prototype.replace with the same pattern.
Quick reference
\d \w \s | digit, word character, whitespace (capitals negate: \D \W \S) |
|---|---|
. ^ $ | any character, start, end |
* + ? {2,4} | 0+, 1+, 0 or 1, between 2 and 4 (add ? to make lazy) |
[abc] [^abc] | any of / none of a, b, c |
(…) (?:…) (?<name>…) | capture group, non-capturing group, named group |
(?=…) (?!…) | lookahead, negative lookahead |
Example
The pre-filled pattern matches South African phone numbers written as 082 123 4567 or 021-555-0199, capturing the area code as a named group. The replacement ($<area>) $2-$3 reformats them as (082) 123-4567 and (021) 555-0199. The “invalid” number doesn’t match.
Frequently asked questions
Which regex flavour does this use?
JavaScript (ECMAScript) regular expressions, as built into your browser. Most syntax is shared with other languages, but some features — such as possessive quantifiers or certain lookbehind rules — differ in PCRE, Python or .NET.
What do the flags mean?
g = find all matches (not just the first); i = ignore case; m = ^ and $ match at line breaks; s = . also matches line breaks; u = Unicode mode (needed for \p{…} classes and emoji); y = sticky (match only at the current position).
Why did my pattern time out?
Some patterns, such as nested quantifiers like (a+)+, can take exponentially long on certain inputs (“catastrophic backtracking”). The tester runs in a background worker and stops after 1.5 seconds so your browser doesn’t freeze. Simplify the pattern or make it more specific.
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