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Regex Tester — Test Regular Expressions Online
Test regular expressions against any string instantly. Highlights matches, extracts capture groups, and explains common patterns. No account, no server calls.
Invalid regular expression
Common patterns
Test String
Matches
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Enter a pattern and test string to see matches highlighted here.
Capture Groups
Pattern explanation —
The at-use.com Regex Tester runs your pattern against your test string in real time using JavaScript’s native RegExp. Every keystroke updates the match highlights, the match count badge, and the capture groups table simultaneously. Nothing is sent to a server — your pattern and data stay in your browser.
Three flag toggles sit to the right of the pattern field. Global (g) finds every match in the string, not just the first. Case insensitive (i) treats uppercase and lowercase as equivalent. Multiline (m) anchors ^ and $ to line boundaries instead of the whole string. All three can be active simultaneously. The pattern input text turns red and an error banner appears immediately if the expression is syntactically invalid — before you finish typing.
The 20-pattern common library covers the patterns developers reach for most often: email addresses, URLs, IPv4 addresses, ISO 8601 dates, US phone numbers, hex colours, HTML tags, URL slugs, usernames, hashtags, and more. Selecting a pattern loads it into the field, sets the appropriate flags, and opens the explanation panel, which describes what each component of the pattern does in plain English. No server call is needed.
Capture groups are highlighted in blue within each amber match and broken out in a table below the panels. Named groups — written as (?<name>...) — appear with their label in the table header. The match count badge updates live; you can copy the raw match list (one match per line) to the clipboard with the copy button in the right panel header.
What the engine supports
The tool uses JavaScript’s ECMAScript 2015+ regex engine: the standard metacharacters (\d, \w, \s, \b), quantifiers (+, *, ?, {n,m}), anchors (^, $), character classes, numbered and named capture groups, non-capturing groups ((?:...)), lookaheads ((?=...), (?!...)), and lookbehinds ((?<=...), (?<!...)). PCRE-specific syntax such as \K (keep) or possessive quantifiers (++) is not supported because it is not part of the ECMAScript standard.
Frequently Asked Questions
What is a regex tester?
A regex tester lets you write a regular expression and test it against a string in real time. Matches are highlighted inline, capture groups are broken out into a table, and common pattern explanations help you understand what each part of the expression does. This tool runs entirely in your browser — nothing is sent to a server.
What regex engine does this use?
The tool uses JavaScript's native RegExp engine, which follows the ECMAScript 2015+ regex spec. It supports the standard metacharacters, quantifiers, anchors, character classes, capture groups (named and numbered), lookaheads, and lookbehinds. PCRE-specific syntax like \K (keep) or possessive quantifiers (++) are not supported.
What do the flags do?
Three flags are available. Global (g) finds all matches in the test string instead of stopping at the first one. Case insensitive (i) makes the match ignore uppercase and lowercase distinctions. Multiline (m) changes ^ and $ to match the start and end of each line rather than the entire string. You can combine all three.
What are capture groups and how do I use them?
A capture group is a part of the pattern wrapped in parentheses that captures the matched text separately from the full match. For example, in (\d{4})-(\d{2})-(\d{2}), three groups capture the year, month, and day independently. The capture groups table shows each group's value per match. Named groups use the syntax (?<name>...) and display their label in the table header.
Is my test string sent to a server?
No. All regex matching runs entirely in your browser using JavaScript's built-in RegExp. Your pattern and test string never leave your device. The tool works offline after the page first loads.
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