Regex Cheat Sheet

Regular expression syntax — anchors, quantifiers, groups, lookaheads, flags, and patterns

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What is Regex Cheat Sheet?

Regex Cheat Sheet is a searchable reference for regular expression syntax covering JavaScript, Python re, and POSIX extended flavors. It organizes patterns by category — anchors, character classes, quantifiers, groups, lookarounds, and flags — so you can scan to the syntax you need rather than reading a full tutorial.

Regular expressions are one of the highest-leverage tools in a developer's toolkit, yet the syntax is dense enough that even experienced engineers keep a reference open. Forgetting whether \b matches a word boundary or backspace, or whether {2,} is greedy by default, or how to write a non-capturing group — this page answers those questions in seconds.

The sheet covers zero-width assertions like positive lookahead, negative lookbehind, named capture groups, backreferences, and the most-used flags (i, g, m, s). It also covers the differences between JavaScript and Python behavior for multiline and dotall modes.

How to Use

  1. Use Ctrl+F / Cmd+F to search for a specific token like (?: or 'lookahead'.
  2. Read the Pattern column for the exact syntax and the Description column for what it matches.
  3. Check the Example column for a concrete string that illustrates the pattern.
  4. Cross-reference the Flavor notes — some syntax (e.g., \A) works in Python but not JavaScript.
  5. Open the Regex Tester tool alongside this page to test patterns as you look them up.

Examples

Pattern: ^\d{4}-\d{2}-\d{2}$

Result: Matches a full ISO date string like 2024-03-15 — anchors ensure no extra characters

Positive lookbehind for dollar sign

Result: Matches a dollar amount only when preceded by $, without capturing the $ character

Pattern: \b[A-Z][a-z]+(?:\s[A-Z][a-z]+)+\b

Result: Matches a proper name of two or more capitalized words like 'John Smith'

Frequently Asked Questions

What is the difference between greedy and lazy quantifiers?

.* is greedy — matches as many characters as possible. .*? is lazy — matches as few. Use lazy quantifiers to match the shortest string between two delimiters.

How do I match a literal dot?

Escape it: \.. An unescaped dot matches any character except newline. Inside character classes like [.], the dot is literal without escaping.

What is a non-capturing group?

(?:...) groups expressions without creating a capture group. This is faster than (...) and keeps match.groups() clean.

Does JavaScript support lookbehind?

Yes, since ES2018. Positive and negative lookbehind are supported in Chrome 62+, Firefox 78+, Safari 16.4+, and Node.js 10+.

How do I make a regex case-insensitive?

Add the i flag: /pattern/i in JavaScript or re.compile('pattern', re.IGNORECASE) in Python.

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