Free Regex Tester & Expression Builder Online — Test, Match & Debug

Regex Pattern

//g

Flags

Test String

Match Preview

Matches will be highlighted here…

Matches & Capture Groups

Matches will appear here…

What is Regular Expression (Regex)?

A regular expression (or regex) is a sequence of characters that defines a search pattern. Regex engines scan text and find substrings that conform to the rules you specify — from simple word searches to complex email validation patterns.

Regular expressions are supported in virtually every programming language — JavaScript, Python, Java, Go, Rust, PHP — and in command-line tools like grep, sed, and awk. They are indispensable for form validation, log parsing, text replacement, and data extraction tasks.

This tool uses JavaScript's native RegExp engine so results are 100% accurate for JS/TypeScript environments. All evaluation happens client-side — your data never leaves your browser.

How to Use the Regex Tester

1Enter your regex pattern in the Pattern field (without the surrounding slashes — they're shown automatically).
2Toggle the flags you need: g (global) finds all matches, i ignores letter case, m makes ^ and $ match line boundaries.
3Paste or type the text you want to search in the Test String textarea.
4Matches are highlighted in real time in the Match Preview panel and listed with their capture groups in the panel on the right.
5If your pattern is invalid (e.g., an unclosed parenthesis), a red error message appears inline — the rest of the page stays stable.
6Click Copy Expression to copy the full /pattern/flags string to your clipboard.

Regex Cheat Sheet

Use this quick-reference table for the most commonly used regex tokens and quantifiers.

Character Classes & Anchors

Token	Description	Example
.	Any character except newline	c.t → cat, cot, cut
\w	Word character [a-zA-Z0-9_]	\w+ → hello, foo_1
\d	Digit [0-9]	\d{3} → 123
\s	Whitespace (space, tab, newline)	\s+ → (spaces)
\W / \D / \S	Negated versions of above	\D → a, !, @
[abc]	Character class — matches a, b, or c	[aeiou] → vowels
[^abc]	Negated class — matches anything except a, b, c	[^0-9] → non-digits
^	Start of string (or line with m flag)	^Hello
$	End of string (or line with m flag)	world$
\b	Word boundary	\bcat\b

Quantifiers & Groups

Token	Description	Example
*	0 or more	\w*
+	1 or more	\d+
?	0 or 1 (optional)	colou?r
{n}	Exactly n times	\d{4}
{n,m}	Between n and m times	\w{3,8}
*? / +?	Lazy (non-greedy)	<.+?>
(abc)	Capturing group — $1	(\w+)@
(?:abc)	Non-capturing group	(?:foo\|bar)
(?<name>...)	Named capturing group	(?<year>\d{4})
a\|b	Alternation — a or b	cat\|dog

FAQ

Is my test text private and secure?

Yes, completely. All regex evaluation runs 100% in your browser using JavaScript's native RegExp engine. Your pattern and test string are never sent to any server. You can even go offline after the page loads and the tool continues to work.

Why does my pattern work differently in Python or Java?

Different languages implement slightly different regex flavors. This tool uses the JavaScript/ECMAScript regex engine. Notable differences: JavaScript does not support lookbehind in older engines, uses \b differently from PCRE, and does not support possessive quantifiers. For Python, use regex101.com's Python flavor option.

What does the 'g' (global) flag do?

Without the global flag, the regex engine stops after the first match. With the 'g' flag enabled, it continues scanning and returns all non-overlapping matches in the test string — which is what most search-and-replace operations need.