Regular Expressions for Beginners: A Guide with Examples

Regular expressions (regex) are a powerful tool for searching and processing text. They may seem complex, but the basic concepts are quite simple. This guide will help you get started.

What are Regular Expressions?

A regular expression is a pattern for searching text. For instance, the expression \d{3}-\d{2}-\d{2} will find all phone numbers in the format 123-45-67.

Basic Syntax

Characters

• . — any single character
• \d — a digit (0-9)
• \w — a letter, digit, or underscore
• \s — a whitespace character (space, tab)
• [abc] — one of the characters a, b, or c
• [^abc] — any character except a, b, c

Quantifiers

• * — 0 or more repetitions
• + — 1 or more repetitions
• ? — 0 or 1 repetition
• {3} — exactly 3 repetitions
• {2,5} — from 2 to 5 repetitions

Anchors

• ^ — start of a line
• $ — end of a line
• \b — word boundary

Practical Examples

Email Validation

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

Phone (Ukraine)

^\+?380\d{9}$

URL Address

^https?:\/\/[-\w.]+\.[a-z]{2,}(\/\S*)?$

IP Address (IPv4)

^(\d{1,3}\.){3}\d{1,3}$

Date (DD.MM.YYYY)

^(0[1-9]|[12]\d|3[01])\.(0[1-9]|1[0-2])\.\d{4}$

Testing Regular Expressions

Use Regex Tester Xuvero to test your expressions in real-time. The tool highlights matches and shows capture groups.

Regex in Different Programming Languages

JavaScript

const pattern = /^\d{3}-\d{2}-\d{2}$/; pattern.test("123-45-67"); // true

Python

import re re.match(r'^\d{3}-\d{2}-\d{2}$', "123-45-67") # Match object

PHP

preg_match('/^\d{3}-\d{2}-\d{2}$/', "123-45-67"); // 1

Common Mistakes

1. Greediness — .* will capture as much as possible. Use .*? for minimal match
2. Unescaped Special Characters — a dot . without \ matches any character
3. Missing Anchors — without ^ and $, the expression will find partial matches