What is a regular expression?

A regular expression (regex) is a pattern used to match and manipulate text. It consists of characters and special symbols that define a search pattern, used in programming and text processing.

What do regex flags do?

Flags modify how a regex pattern is interpreted. Common flags: g (global - find all matches), i (case-insensitive), m (multiline), s (dotall), u (unicode).

What are capture groups?

Capture groups are parts of a regex pattern enclosed in parentheses that capture matched text for later use. They're numbered from left to right and can be referenced with backslash notation.

How do I match email addresses?

A basic email pattern is /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/. However, complete RFC-compliant email validation is very complex.

What is the difference between greedy and lazy matching?

Greedy matching (*,+,{}) matches as much as possible. Lazy matching (*?,+?,{}?) matches as little as possible. Use lazy quantifiers when you want to match smaller portions of text.

Can I use lookaheads and lookbehinds?

Yes. Lookahead (?=...) asserts that what follows matches the pattern. Lookbehind (?<=...) asserts that what precedes matches. They don't consume characters.

Regex Tester & Debugger

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Regex Pattern

Regular Expression:

Flags:

Global (g)

Case-insensitive (i)

Multiline (m)

Dotall (s)

Unicode (u)

Test String:

Quick Patterns:

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Enter Regex Pattern:

Character Classes

Pattern	Description	Example
.	Any character except newline	a.c matches "abc", "adc"
\d	Digit (0-9)	\d+ matches "123"
\w	Word character (a-z, A-Z, 0-9, _)	\w+ matches "hello_123"
\s	Whitespace (space, tab, newline)	\s+ matches spaces
[abc]	Character set	[abc] matches "a", "b", or "c"
[^abc]	Negated character set	[^abc] matches any char except a,b,c

Quantifiers

Pattern	Description	Example
*	0 or more (greedy)	ab*c matches "ac", "abc", "abbc"
+	1 or more (greedy)	ab+c matches "abc", "abbc"
?	0 or 1 (greedy)	ab?c matches "ac", "abc"
{n}	Exactly n times	a{3} matches "aaa"
{n,m}	Between n and m times	a{2,4} matches "aa", "aaa", "aaaa"
*? +? ?? {n,m}?	Lazy versions	ab*?c matches "ac", "abc" (shortest match)

Anchors & Boundaries

Pattern	Description	Example
^	Start of string/line	^hello matches "hello world"
$	End of string/line	world$ matches "hello world"
\b	Word boundary	\bword\b matches "word" as whole word
\B	Non-word boundary	\Bond\B matches "mond" in "mond"

Groups & Lookarounds

Pattern	Description	Example
(abc)	Capture group	(abc)+ captures "abc"
(?:abc)	Non-capturing group	(?:abc)+ doesn't capture
(?=abc)	Positive lookahead	\w+(?=ing) matches "walk" in "walking"
(?!abc)	Negative lookahead	\w+(?!ing) matches "walk" in "walk"
(?<=abc)	Positive lookbehind	(?<=re)start matches "start" in "restart"
(?	Negative lookbehind	(?

Escape Sequences

Sequence	Description
\\	Backslash
\/	Forward slash
\n	Newline
\t	Tab
\r	Carriage return
\0	Null character

Pre-built Pattern Library

Click "Test" to load a pattern into the Regex Tester. Click "Copy" to copy to clipboard.

Regex Tester & Debugger

Frequently Asked Questions