Which regex flavor does this tool support?

Our tool currently supports the JavaScript (ECMAScript) regular expression flavor, as it runs directly in your web browser.

What are regex flags?

Flags are optional parameters that modify the behavior of the regex. Common flags include 'g' (global, find all matches), 'i' (case-insensitive), and 'm' (multiline).

Yes. All regex evaluation happens locally in your browser. Your test strings and regex patterns are never sent to our servers.

What regular expression flavors are supported?

The tool tests against JavaScript's RegExp engine. Most standard regex features like groups, lookaheads, character classes, and quantifiers are supported.

How do I match an email address with regex?

A common pattern is /^[\w.-]+@[\w.-]+\.[a-zA-Z]{2,}$/. However, email validation with regex is complex — for production, consider using a dedicated validation library.

What do the flags g, i, and m mean?

g (global) finds all matches, not just the first. i (case-insensitive) ignores case. m (multiline) makes ^ and $ match start/end of each line instead of the whole string.

How do I test a regex with capture groups?

Wrap parts of your pattern in parentheses, e.g., /(\d{4})-(\d{2})-(\d{2})/. The tool highlights each group and shows the captured values separately.

📝

Regular Expression Syntax Guide

Master the art of pattern matching. This comprehensive guide covers every regex syntax element from basic metacharacters to advanced lookaround assertions.

Metacharacters

Char	Description	Example
.	Matches any character except newline	a.c → abc, a1c, a-c
^	Matches start of string/line	^Hello → 'Hello world'
$	Matches end of string/line	world$ → 'Hello world'
\d	Matches any digit [0-9]	\d{3} → 123, 456
\w	Matches word character [a-zA-Z0-9_]	\w+ → hello_123
\s	Matches whitespace (space, tab, newline)	a\sb → 'a b'
\b	Matches word boundary	\bcat\b → 'the cat sat'
\D	Matches non-digit	\D+ → 'abc'
\W	Matches non-word character	\W → @, #, !
\S	Matches non-whitespace	\S+ → 'hello'

Quantifiers (How Many)

Syntax	Meaning	Example
*	0 or more	ab*c → ac, abc, abbc
+	1 or more	ab+c → abc, abbc (not ac)
?	0 or 1 (optional)	colou?r → color, colour
{n}	Exactly n times	\d{4} → 2024
{n,}	n or more times	\d{2,} → 10, 100, 1000
{n,m}	Between n and m times	\d{2,4} → 10, 100, 1000
*?	0 or more (lazy)	<.*?> → <b> (not <b>text</b>)
+?	1 or more (lazy)	Minimal match

💡 Greedy vs Lazy: By default, quantifiers are greedy (match as much as possible). Add ? to make them lazy (match as little as possible).

Character Classes

[abc]

Match any single character: a, b, or c

[^abc]

Match any character EXCEPT a, b, or c

[a-z]

Match any lowercase letter (range)

[0-9A-Fa-f]

Match any hexadecimal digit

Groups & Assertions

Syntax	Type	Example
(abc)	Capturing group	(\d{3})-(\d{4}) → captures 123, 4567
(?:abc)	Non-capturing group	(?:ab)+ → abab (no capture)
(?<name>)	Named capture group	(?<year>\d{4})
(?=abc)	Positive lookahead	\d(?=px) → '10px' matches 10
(?!abc)	Negative lookahead	\d(?!px) → '10em' matches 10
(?<=abc)	Positive lookbehind	(?<=\$)\d+ → '$100' matches 100
(?<!abc)	Negative lookbehind	(?<!\$)\d+ → '100' (no $)
\|	Alternation (OR)	cat\|dog → cat or dog

✨ Lookahead/Lookbehind: These "zero-width assertions" check for patterns without including them in the match. Perfect for validating context without consuming characters.