🧪 Regex Tester ☆

Overview

Regex Tester lets you test regular expressions as you type. Enter a pattern, pick your flags (g, i, m, s, u), paste the test text and instantly see every match highlighted in place, with the match list showing positions and capture group values. Invalid patterns produce clear error messages instead of silent failures. A quick-reference cheat sheet for \d, \w, \s, quantifiers, sets, groups and anchors is included. The regex engine runs entirely in your browser, neither the pattern nor the text is uploaded, which matters for real-world data like logs, emails and CSVs.

How the tester evaluates patterns

The pattern is compiled by the browser's own JavaScript regex engine, so the behavior you see is exactly what your code will get, including edge cases like backtracking, greedy versus lazy quantifiers and Unicode with the u flag. Matches are highlighted in place, and the list shows each match with its index and capture groups. Syntax errors are reported with a readable message instead of failing silently, and the cheat sheet covers classes, anchors, quantifiers and groups.

How it works and what it supports

Property	Regex tester behavior
Engine	Native JavaScript RegExp (same as your production code)
Flags	g, i, m, s, u, y as supported by the browser
Output	Inline highlighting, match list with positions and groups
Errors	Clear messages for invalid patterns
Reference	Cheat sheet for classes, sets, groups and anchors
Processing	100% client-side; pattern and text never uploaded
Cost	Free, no account, no ads

Privacy: patterns and data stay in the browser

Regex tests often run against real log lines, emails and CSV rows. Everything is evaluated locally: nothing is transmitted, stored or logged, and the page works offline after loading.

Writing safer patterns

• Prefer specific classes over . to avoid accidental matches.
• Anchor with ^ and $ when validating whole strings.
• Beware nested quantifiers. They cause catastrophic backtracking on long input.
• Test with real-world data, not just a happy-path sample.
• Escape user input before inserting it into a pattern.

Related: browse the developer tools for JSON, JWT, UUID and URL utilities.

Catastrophic backtracking and regex performance

A regular expression that matches correctly on a short string can still hang on a long one, and the usual culprit is backtracking. Engines try alternatives when a match fails, and nested quantifiers multiply those alternatives. A pattern like (a+)+b against a string of many a's with no b forces the engine to explore an exponential number of ways to split the input, which is the classic denial-of-service shape known as ReDoS. The symptoms are easy to recognize: a test that finishes instantly on ten characters and never returns on thirty. Three habits prevent it. Prefer specific character classes to broad dots, and make quantifiers as constrained as the data allows, [a-z]{1,10} instead of .*. Use anchors (^ and $) and word boundaries so the engine knows where a match may start, which removes huge amounts of pointless searching. Where the language supports them, atomic groups or possessive quantifiers tell the engine not to revisit a decision it already made. When testing, always try a valid example, an invalid example and a deliberately long input that almost matches, because the near-miss is what triggers backtracking. The tester runs in your browser with no server round trip, so you can experiment freely with production patterns without sending them anywhere.