Why do I need to escape special characters in regex?

Characters like . * + ? [ ] ( ) { } \ ^ $ | have special meanings in regex. To match them literally, escape with backslash: \. matches a period, \* matches an asterisk. In JavaScript strings, double-escape: '\\.' because \ is also a string escape character.

What is the difference between .* and .*?

.* is greedy - it matches as much as possible. .*? is lazy/non-greedy - it matches as little as possible. Example: / / on ' ' matches ' ' (everything). / / matches ' ' then ' ' (minimum each time). Use lazy matching for nested structures.

How do I make my regex case-insensitive?

Add the 'i' flag: /hello/i matches 'Hello', 'HELLO', 'hElLo'. In JavaScript: new RegExp('hello', 'i'). You can combine flags: /hello/gi for global case-insensitive matching. Some patterns need explicit alternatives: /[a-zA-Z]/ matches letters in either case without the flag.

Regular Expressions: A Beginner's Guide

Regular Expressions Basics

Regular expressions (regex) are patterns for matching text. They're powerful but can be intimidating at first.

Simple Patterns

// Literal match /hello/ // Matches "hello" // Case insensitive /hello/i // Matches "Hello", "HELLO", etc.

// Global (find all) /hello/g // Finds all occurrences

Character Classes

/[abc]/        // Matches a, b, or c
/[a-z]/        // Matches any lowercase letter
/[A-Z]/        // Matches any uppercase letter
/[0-9]/        // Matches any digit
/[a-zA-Z0-9]/  // Matches any alphanumeric
// Negation
/[^abc]/       // Matches anything EXCEPT a, b, c

Shorthand Classes

\d             // Digit [0-9]
\D             // Non-digit [^0-9]
\w             // Word char [a-zA-Z0-9_]
\W             // Non-word char
\s             // Whitespace
\S             // Non-whitespace
.              // Any character (except newline)

Quantifiers

/a*/           // Zero or more a's
/a+/           // One or more a's
/a?/           // Zero or one a
/a{3}/         // Exactly 3 a's
/a{2,4}/       // 2 to 4 a's
/a{2,}/        // 2 or more a's

Anchors

/^hello/       // Starts with "hello"
/world$/       // Ends with "world"
/^exact$/      // Exactly "exact"
/\bhello\b/    // "hello" as whole word

Common Patterns

// Email (simple)
/^[^\s@]+@[^\s@]+\.[^\s@]+$/
// URL
/https?:\/\/[^\s]+/
// Phone (US)
/\d{3}-\d{3}-\d{4}/
// Date (YYYY-MM-DD)
/\d{4}-\d{2}-\d{2}/