Regular Expressions (Regex) Beginner's Guide

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What is Regex?

A regular expression (regex or regexp) is a sequence of characters that defines a search pattern. Regex is used to search, match, extract, validate, and replace text. It is supported in virtually every programming language and is one of the most powerful tools in a developer's toolkit.

For example, the regex \d{3}-\d{4} matches phone number patterns like "555-1234". Once you understand the building blocks, you can write patterns to match almost any text structure.

Basic Syntax: Character Classes

Pattern	Matches
.	Any character except newline
\d	Any digit (0-9)
\D	Any non-digit
\w	Word character (a-z, A-Z, 0-9, _)
\W	Non-word character
\s	Whitespace (space, tab, newline)
\S	Non-whitespace
[abc]	Any of: a, b, or c
[^abc]	Any character except a, b, c
[a-z]	Any lowercase letter
[A-Z]	Any uppercase letter
[0-9]	Any digit (same as \d)

Quantifiers

Quantifier	Meaning
*	Zero or more times
+	One or more times
?	Zero or one time (optional)
{n}	Exactly n times
{n,}	At least n times
{n,m}	Between n and m times
*? +? ??	Lazy (non-greedy) versions — match as few as possible

Anchors & Boundaries

^

Start of string (or line with m flag)

$

End of string (or line with m flag)

\b

Word boundary — position between a word char and non-word char

\B

Non-word boundary

Groups & Alternation

(abc)

Capturing group — captures the matched text for later use

(?:abc)

Non-capturing group — groups without capturing

(?<name>abc)

Named capturing group — access by name

a|b

Alternation — matches a or b

(?=abc)

Positive lookahead — matches if followed by abc

(?!abc)

Negative lookahead — matches if NOT followed by abc

Practical Regex Examples

Email validation

^[a-zA-Z0-9._%+\-]+@[a-zA-Z0-9.\-]+\.[a-zA-Z]{2,}$

Matches standard email addresses. Not 100% RFC-compliant but covers 99% of real emails.

URL matching

https?:\/\/[^\s/$.?#].[^\s]*

Matches http and https URLs.

Phone number (US)

^\+?1?[\s.-]?$?\d{3}$?[\s.-]?\d{3}[\s.-]?\d{4}$

Matches US phone numbers in various formats.

Strong password

^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$

Requires lowercase, uppercase, digit, special char, min 8 chars.

IPv4 address

^((25[0-5]|2[0-4]\d|[01]?\d\d?)\.){3}(25[0-5]|2[0-4]\d|[01]?\d\d?)$

Validates IPv4 addresses (0.0.0.0 to 255.255.255.255).

Hex color code

^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$

Matches 3 or 6 digit hex color codes like #FFF or #FF5733.

Regex Flags

g

Global — find all matches, not just the first

i

Case-insensitive — match regardless of letter case

m

Multiline — ^ and $ match start/end of each line

s

DotAll — dot (.) matches newline characters too

u

Unicode — enables full Unicode support

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