Regex

2026-09-14

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7 min read

JavaScript Regular Expressions Cheat Sheet & Matcher Guide

Master character classes, lookaheads, capture groups, and regex performance optimization.

Regular expressions (Regex) provide powerful pattern matching and textual search-and-replace capabilities across all programming languages.

1. Core Regex Character Classes - `\d`: Match any numeric digit (0-9). - `\w`: Match any word character (A-Z, a-z, 0-9, `_`). - `\s`: Match any whitespace character (space, tab, newline). - `.`: Match any character except newlines. - `[A-Z]`: Match any uppercase character range.

2. Regex Flags in JavaScript - `g` (Global): Match all occurrences throughout the target string. - `i` (Case-Insensitive): Ignore character casing differences. - `m` (Multiline): Anchors `^` and `$` match line beginnings and ends. - `s` (DotAll): Allows dot (`.`) to match newline characters.

3. Lookaheads and Lookbehinds - Positive Lookahead (`(?=...)`): Asserts that the expression matches ahead without consuming characters. - Negative Lookahead (`(?!...)`): Asserts that the expression does NOT match ahead.

Interactive Tools for This Guide

Regex Tester

Test and debug JavaScript regular expressions with real-time match highlighting.

HTML Formatter

Format, clean, and beautify raw HTML markup with consistent indentation.

CSS Formatter

Format and beautify CSS stylesheets with clean rule indentation.

Frequently Asked Questions

What is catastrophic backtracking in regex?

Catastrophic backtracking occurs when nested quantifiers (like (a+)+) match ambiguous strings, forcing the regex engine to test exponential permutations and freezing the CPU thread.