Regex Cheatsheet: Patterns, Anchors & Common Examples

regex-cheatsheet.md

Regex Cheatsheet: Patterns, Anchors & Common Examples

A regular-expression reference that works across most engines (PCRE, JavaScript, Python, Java, Go). Character classes, quantifiers, groups, lookarounds, plus copy-paste patterns for the things people actually match: email, URL, dates, IPs.

Character classes

Pattern	Matches
.	any character except newline
\d	digit [0-9]
\D	non-digit
\w	word char [A-Za-z0-9_]
\W	non-word char
\s	whitespace (space, tab, newline)
\S	non-whitespace
[abc]	a, b, or c
[^abc]	NOT a, b, or c
[a-z]	range a to z
[a-zA-Z0-9]	alphanumeric

Anchors & boundaries

Pattern	Matches
^	start of string/line
$	end of string/line
\b	word boundary
\B	non-word boundary
\A \Z	start / end of whole string (PCRE/Python)

Quantifiers

Pattern	Means
*	0 or more
+	1 or more
?	0 or 1 (optional)
{3}	exactly 3
{2,5}	between 2 and 5
{2,}	2 or more
*? +?	lazy (as few as possible)

Groups & alternation

(abc)         capturing group
(?:abc)       non-capturing group
(?<name>abc)  named group
a|b           a OR b
\1            backreference to group 1

Lookarounds

Pattern	Means
(?=foo)	positive lookahead (followed by foo)
(?!foo)	negative lookahead (NOT followed by foo)
(?<=foo)	positive lookbehind (preceded by foo)
(?<!foo)	negative lookbehind

Flags

Flag	Effect
i	case-insensitive
g	global (all matches, JS)
m	multiline (^/$ match each line)
s	dotall (. matches newline)

Copy-paste patterns

# Email (practical, not RFC-perfect)
[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}

# URL (http/https)
https?://[^\s/$.?#].[^\s]*

# IPv4 address
\b(?:\d{1,3}\.){3}\d{1,3}\b

# Date YYYY-MM-DD
\d{4}-\d{2}-\d{2}

# Time HH:MM (24h)
([01]\d|2[0-3]):[0-5]\d

# Hex color
#(?:[0-9a-fA-F]{3}|[0-9a-fA-F]{6})\b

# US phone
\(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}

# Strong password (8+, upper, lower, digit, symbol)
(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[^\w\s]).{8,}

# Leading/trailing whitespace
^\s+|\s+$

# Duplicate words
\b(\w+)\s+\1\b

Usage by language

// JavaScript
"a1b2".replace(/\d/g, "#");          // "a#b#"
const m = str.match(/(\d{4})-(\d{2})/);

# Python
import re
re.findall(r"\d+", "a1b22")          # ['1', '22']
re.sub(r"\s+", " ", text)            # collapse whitespace
m = re.search(r"(?P<year>\d{4})", s); m.group("year")

# grep (extended)
grep -E "[0-9]{3}-[0-9]{4}" file

Quick reference table

Want	Pattern
One or more digits	\d+
Optional char	x?
Word boundary	\b
Non-capturing group	(?:...)
Lazy match	*?
Case-insensitive	flag i
Email	[\w.%+-]+@[\w.-]+\.[A-Za-z]{2,}
IPv4	(?:\d{1,3}\.){3}\d{1,3}

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