Regex Tester — Free Real-Time Regular Expression Highlighter & Matcher
Test regular expressions with real-time match highlighting. Supports flags, capture group display, replace mode, and a built-in cheatsheet — all in your browser.
Frequently Asked Questions
Q. What is a regular expression (Regex)?
A special language for expressing string patterns, used for text search, extraction, and replacement. Essential for programming, log analysis, and data processing.
Q. What are flags?
g (global search), i (case insensitive), m (multiline), and s (dotAll — dot matches newlines). Multiple flags can be combined.
Q. What are capture groups?
Parts of the pattern enclosed in parentheses (). Used to extract specific portions of a match. Referenced as $1, $2, etc.
Q. How do I use Replace mode?
Enable Replace mode to see the result of substituting matched strings with your replacement text. Use $1–$9 to reference capture groups.
Q. Is there a cheatsheet for learning regex?
Yes — click the Cheatsheet button to instantly view 15 commonly used patterns with descriptions.
Q. What happens if I enter an invalid regex?
An error message appears immediately below the input. Matching is paused while the error exists.
How to Use
Type your regular expression in the pattern input. (e.g., \d+)
Choose g, i, m, or s flags as needed.
Input the string you want to test against the pattern.
Matches are highlighted; capture groups and replace results are shown.
Expert Knowledge: Regex Tester — Free Real-Time Regular Expression Highlighter & Matcher
Regular expressions trace their origins to mathematician Stephen Kleene's development of regular language theory in 1956. Ken Thompson first implemented them in the QED editor in 1968, and they were popularized through the Unix grep tool. JavaScript's RegExp follows the ECMA-262 standard; ES2018 added look-behind assertions (?<=) and named capture groups (?<name>). Complex regexes can introduce ReDoS (Regular Expression Denial of Service) vulnerabilities — avoid patterns with excessive backtracking.
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