Regex Cheat Sheet: Patterns, Syntax & Examples

What are regular expressions?

Regular expressions (regex or regexp) are patterns used to match character combinations in strings. They are a powerful tool for searching, validating, and manipulating text. Every major programming language supports regex: JavaScript, Python, Java, PHP, Go, and more.

Basic syntax

Character classes

• . — Matches any character (except newline)
• \d — Matches any digit (0-9)
• \D — Matches any non-digit
• \w — Matches any word character (a-z, A-Z, 0-9, _)
• \W — Matches any non-word character
• \s — Matches any whitespace (space, tab, newline)
• \S — Matches any non-whitespace
• [abc] — Matches a, b, or c
• [^abc] — Matches anything except a, b, or c
• [a-z] — Matches any lowercase letter

Quantifiers

• * — 0 or more occurrences
• + — 1 or more occurrences
• ? — 0 or 1 occurrence (optional)
• {3} — Exactly 3 occurrences
• {2,5} — Between 2 and 5 occurrences
• {3,} — 3 or more occurrences

Anchors

• ^ — Start of string (or line in multiline mode)
• $ — End of string (or line in multiline mode)
• \b — Word boundary

Groups and alternation

• (abc) — Capturing group
• (?:abc) — Non-capturing group
• a|b — Matches a or b (alternation)
• (?=abc) — Positive lookahead
• (?!abc) — Negative lookahead

Common regex patterns

Email validation

^[\w.-]+@[\w.-]+\.[a-zA-Z]{2,}$

Matches most common email formats. Note: a fully RFC-compliant email regex is extremely complex; this pattern covers 99% of real-world emails.

URL validation

^https?:\/\/[\w.-]+\.[a-zA-Z]{2,}[\/\w.-]*$

Phone number (international)

^\+?[1-9]\d{6,14}$

Date format (YYYY-MM-DD)

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

IP address (IPv4)

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

Strong password

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

Requires: minimum 8 characters, at least one uppercase, one lowercase, one digit, and one special character.

Regex flags

• g — Global: match all occurrences, not just the first
• i — Case-insensitive matching
• m — Multiline: ^ and $ match line boundaries
• s — Dotall: . matches newlines too
• u — Unicode: full Unicode support

Testing your regex

Always test your regular expressions before using them in production. Try our free regex tester to validate patterns in real time with instant highlighting of matches.

Performance tips

• Be specific: [0-9] is faster than .* for matching digits
• Avoid catastrophic backtracking: patterns like (a+)+ can cause exponential time complexity
• Use non-capturing groups when you don't need the match: (?:...) instead of (...)
• Anchor your patterns: use ^ and $ when validating entire strings