What is a regular expression (regex)?

A regular expression (regex) is a sequence of characters that defines a search pattern. It is used for string matching, validation, extraction, and replacement in programming and text processing.

How do I test a regex pattern online?

Enter your regex pattern in the pattern field, type or paste your test string below, and RegexLab will instantly highlight all matches in real time. You can also view capture groups and match positions.

What are capture groups in regex?

Capture groups are portions of a regex pattern enclosed in parentheses (). They allow you to extract specific parts of a match. For example, in the pattern (\d{3})-(\d{4}), Group 1 captures the first three digits and Group 2 captures the last four.

What flags does JavaScript regex support?

JavaScript supports several regex flags: g (global – find all matches), i (case-insensitive), m (multiline – ^ and $ match line boundaries), s (dotAll – dot matches newlines), and u (unicode – enables full Unicode matching).

Is regex matching case sensitive by default?

Yes, by default regex matching is case sensitive. To make it case insensitive, add the 'i' flag to your pattern. In RegexLab, simply check the 'i' flag checkbox next to the pattern input.

RegexLab – Free Online Regex Tester, Builder & Debugger

Free Online Regex Tester & Builder

Test and debug your JavaScript regular expressions in real time with live match highlighting, capture groups, and an instant plain-English explanation.

Regular Expression

Flags: g i m s u

/ / g

0.00ms

Test String

Replace Mode Enable

Enter a regex and test string to see matches

Common Patterns

What is a Regular Expression?

A regular expression (regex or regexp) is a powerful sequence of characters that defines a search pattern. Originally rooted in formal language theory, regex has become an indispensable tool for developers, data analysts, and system administrators worldwide. Regular expressions are used for pattern matching within strings — enabling tasks like input validation (emails, phone numbers, passwords), text searching and extraction, find-and-replace operations, and data parsing. In JavaScript, regex patterns are enclosed between forward slashes (/pattern/flags) and can be tested with methods like .test(), .match(), and .replace(). While regex syntax can appear cryptic at first, learning the fundamentals — character classes, quantifiers, anchors, and groups — unlocks the ability to process text with surgical precision. RegexLab makes this learning process interactive by providing instant feedback as you type.

How to Use RegexLab

1 Enter your regex — Type your regular expression pattern in the input field at the top. Toggle flags (g, i, m, s, u) as needed.
2 Paste your test string — Add the text you want to test against in the test string area. Matches highlight instantly.
3 Review matches — Check the Matches panel for details: each match's value, position, and capture groups.
4 Read the explanation — Switch to the Explanation tab for a plain-English breakdown of your pattern.
5 Use Replace mode — Enable it to preview string replacements with your regex live. Use $1, $2 for group references.

Regex Cheat Sheet

Anchors & Boundaries

Symbol	Meaning	Example
^	Start of string	^Hello
$	End of string	world$
\b	Word boundary	\bword\b
\B	Non-word boundary	\Bend

Quantifiers

Symbol	Meaning	Example
*	0 or more	ab*c
+	1 or more	ab+c
?	0 or 1	colou?r
{n}	Exactly n times	\d{3}
{n,m}	Between n and m	\d{2,4}

Character Classes

Symbol	Meaning	Example
.	Any character (except newline)	a.c
\d	Digit [0-9]	\d+
\w	Word char [a-zA-Z0-9_]	\w+
\s	Whitespace	\s+
[abc]	Any of a, b, c	[aeiou]
[^abc]	Not a, b, or c	[^0-9]

Groups & Assertions

Symbol	Meaning	Example
(abc)	Capture group	(\d+)-(\d+)
(?:abc)	Non-capture group	(?:ab)+
a\|b	Alternation (or)	cat\|dog
(?=abc)	Positive lookahead	\d(?=px)
(?!abc)	Negative lookahead	\d(?!px)
(?<=abc)	Positive lookbehind	(?<=\$)\d+

Frequently Asked Questions

Capture groups are sections of a regex pattern enclosed in parentheses (). They allow you to extract specific portions of a match. For example, in the pattern (\d{3})-(\d{4}), Group 1 captures the first three digits and Group 2 captures the last four. In replacements, you reference groups with $1, $2, etc.

JavaScript supports several regex flags: g (global — find all matches, not just the first), i (case-insensitive matching), m (multiline — ^ and $ match line starts/ends), s (dotAll — . matches newlines), and u (unicode — enables full Unicode support and stricter parsing).

Yes. By default, regex patterns are case sensitive — /hello/ will not match "Hello" or "HELLO". To perform a case-insensitive match, add the i flag to your pattern: /hello/i. In RegexLab, simply check the "i" flag checkbox next to the pattern input.