What is a Regular Expression?

Overview

A regular expression is a special pattern of characters used to find, match, or replace text in strings.
Think of it like a smart search took - instead of looking for exact words, it can look for patterns like:

• "all emails"
• "every number"
• "any word that ends with .com"
• "dates like 2025-06-27"

Basic Parts of Regular Expressions

Here are some simple and common elements:

Symbol	Meaning	Example	What it Matches
.	Any one character	a.b	aab, acb, a1b
*	0 or more of the previous thing	go*	g, go, goo, goooooo
+	1 or more of the previous thing	go+	go, goo, but not g
?	0 or 1 of the previous thing	colou?r	color or colour
\d	A digit (0-9)	\d\d\d	123, 456
\w	A word character (letter, number, or _)	\w+	hello123, user_name
\s	A whitespace (space, tab, etc.)	\s+	spaces, tabs, etc.
^	Start of string	^Hello	matches strings that start with Hello
$	End of string	world$	matches strings that end with world
[abc]	One of a, b, or c	gr[ae]y	gray or grey
[^abc]	Not a, b, or c	[^0-9]	anything thats not a digit
(abc)	Grouping	(ha)+	ha, hahaha

Simple Examples

• Match an email: /\w+@\w+\.\w+/ -> matches things like [e-mail]
• Match a phone number: /\d{3}-\d{3}-\d{4}/ -> matches 123-456-7890
• Match a date (YYYY-MM-DD): /\d{4}-\d{2}-\d{2}/ -> matches 2025-06-27