Interactive Regex Cheatsheet

Master Regular Expressions in Real-Time — a searchable, filterable reference for syntax across PCRE, Python, JavaScript, Java, and Go. Copy any snippet with one click.

Browse by Category

Regex Syntax Categories

Jump straight to the pattern family you need. Each card links to a filtered view of the full cheatsheet table below.

Anchors & Boundaries

Match string positions — ^, $, \b, \B, \A, \Z. Essential for validating full lines, trimming whitespace, or locking patterns to word edges.

Quantifiers

Control repetition with *, +, {n,m}, and their lazy variants *?, +?. Includes possessive quantifiers for Java and PCRE.

Character Classes

Shorthand classes \d, \w, \s, POSIX bracket expressions like [[:alpha:]], and custom ranges. Covers Unicode property escapes \p{L} supported in Java, JS (with u flag), and Go.

Groups & Lookarounds

Capturing (…), non-capturing (?:…), named groups (?<name>…), lookaheads (?=…), and lookbehinds (?<=…). Compatibility matrix included for each engine.

Assertions & Flags

Inline modifiers (?i), (?m), (?s), engine-specific flags (PCRE's J for duplicated names, Go's case-insensitive ? prefix), and atomic grouping (?>…).

Common Patterns

Battle-tested recipes: email validation, IPv4/IPv6 matching, URL parsing, date formats (ISO 8601, US, EU), phone numbers, and credit-card Luhn-check patterns ready to paste into your codebase.

Full Reference

Searchable Regex Table

Filter by engine, category, or keyword. Every row is copyable — click the pattern column to grab the snippet instantly.

Pattern	Description	Category	PCRE	Python	JavaScript	Java	Go
`^\d{3}-\d{2}-\d{4}$`	US Social Security Number (XXX-XX-XXXX)	Common Patterns	✅	✅	✅	✅	✅
`\b[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,}\b`	Standard email address (case-insensitive flag required)	Common Patterns	✅	✅	✅	✅	✅
`(?<=\b\w{3})\w+`	Lookbehind: word following a 3-letter word boundary	Groups & Lookarounds	✅	✅ (3.6+)	✅ (ES2018+)	✅	❌
`\p{Lu}\p{Ll}+`	Capitalized word (Unicode-aware)	Character Classes	✅	✅ (regex module)	✅ (with `u` flag)	✅	✅
`(?i)^https?://`	HTTP or HTTPS scheme at line start (inline case-insensitive)	Assertions & Flags	✅	✅	❌ (use `/i` flag)	✅	❌ (prefix with `(?i)` not supported; use `regexp.MustCompile("(?i)https?://")`)
`\d{1,3}(\.\d{1,3}){3}`	IPv4 address (basic structural match; does not validate 0–255 range)	Common Patterns	✅	✅	✅	✅	✅
`(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})`	ISO 8601 date with named capture groups	Groups & Lookarounds	✅	✅	❌	✅	❌
`\w+@\w+\.\w{2,4}`	Simplified email (no special characters, 2–4 TLD)	Common Patterns	✅	✅	✅	✅	✅
`[[:digit:]]{5}(-[[:digit:]]{4})?`	US ZIP code with optional 4-digit extension (POSIX class)	Character Classes	✅	✅ (re module, via `\d` equiv.)	❌	✅	❌
`^.{0,255}$`	String length validation: max 255 characters	Anchors & Boundaries	✅	✅	✅	✅	✅
`(?>\w+\s*)`	Atomic group: consume word + trailing spaces without backtracking	Groups & Lookarounds	✅	❌	❌	✅	❌
`\d{4}(?= \|,\|-)\d{2}\d{2}`	Year followed by separator-aware date digits (lookahead)	Groups & Lookarounds	✅	✅	✅	✅	✅
`[^<>"'\s]+`	Non-whitespace token excluding HTML/quote characters	Character Classes	✅	✅	✅	✅	✅
`\b\d{1,2}:\d{2}(?::\d{2})?(?:[AP]M)?\b`	12-hour or 24-hour time with optional seconds and AM/PM	Common Patterns	✅	✅	✅	✅	✅
`(?m)^# [^\n]+$`	Markdown-level-1 heading (multiline mode)	Assertions & Flags	✅	✅	✅ (with `m` flag)	✅	✅ (with `m` flag)

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