Regular Expression (Regex)

A regular expression (regex) is a formal language for describing text patterns. It uses metacharacters alongside literal characters to specify matching rules: character classes like \d for digits, quantifiers like + for one or more, anchors like ^ for start of string, and groups like (abc) for capturing substrings. Every major programming language includes a regex engine.

Basic Syntax

.     Any character except newline
\d    Digit [0-9]
\w    Word character [a-zA-Z0-9_]
\s    Whitespace
^     Start of string (or line in multiline mode)
$     End of string
*     Zero or more (greedy)
+     One or more (greedy)
?     Zero or one
{n}   Exactly n times
{n,m} Between n and m times
[abc] Character class — matches a, b, or c
[^a]  Negated class — anything except a
(abc) Capturing group
|     Alternation (OR)
\b    Word boundary

Named Groups

const match = "2026-06-15".match(/(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/);
match.groups.year  // "2026"
match.groups.month // "06"
match.groups.day   // "15"

Lookahead and Lookbehind

foo(?=bar)   # "foo" only if followed by "bar"
foo(?!bar)   # "foo" only if NOT followed by "bar"
(?<=\$)\d+   # digits preceded by $
(?<!\$)\d+   # digits NOT preceded by $

Common Patterns

Email:   ^[a-zA-Z0-9._%+\-]+@[a-zA-Z0-9.\-]+\.[a-zA-Z]{2,}$
URL:     https?:\/\/[\w\-.]+\.[a-zA-Z]{2,}(\/[\w\-./?%&=]*)?
Date:    ^\d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])$
HEX:     ^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$
IPv4:    ^(\d{1,3}\.){3}\d{1,3}$

Flags

/pattern/i   Case-insensitive
/pattern/g   Global (find all matches)
/pattern/m   Multiline (^ and $ match line boundaries)
/pattern/s   Dotall (. matches newlines)
/pattern/u   Unicode mode

Test and debug patterns with the Regex Tester Tool.