What does . mean in regex?

A dot (.) matches any single character except a newline. To match a literal dot, escape it: \. Use the s flag (dotall mode) to make . also match newlines.

What is the difference between * and + in regex?

* matches zero or more of the preceding element. + matches one or more. So a* matches '' and 'aaa', while a+ only matches 'a', 'aa', 'aaa' — never empty.

What does \b mean in regex?

\b is a word boundary anchor. It matches the position between a word character (\w) and a non-word character (\W) or the start/end of the string. \bcat\b matches 'cat' but not 'cats' or 'concatenate'.

What is a non-capturing group in regex?

(?:...) groups characters for alternation or quantifiers without creating a capture group. Use it when you need grouping but don't need to reference the match later — it's faster and cleaner than a full capture group.

What is a lookahead in regex?

A lookahead (?=...) asserts that what follows the current position matches the pattern, without consuming characters. \d+(?= dollars) matches a number only if followed by ' dollars'. A negative lookahead (?!...) asserts the opposite.

Regex Reference

Regex Cheat Sheet
Quick Reference for Developers

Every pattern, quantifier, anchor, group and flag — with examples you can copy and test immediately.

Reference·Updated May 2026

Regex cheat sheet for developers: a single-page reference covering everything from basic character classes to lookaheads, with copy-paste patterns for the most common use cases. Each section links to the free regex tester so you can try any pattern instantly.

Character Classes

Pattern	Meaning
.	Any character except newline (add s flag for newline too)
\d	Digit: [0-9]
\D	Non-digit: [^0-9]
\w	Word character: [a-zA-Z0-9_]
\W	Non-word character
\s	Whitespace: space, tab, newline, carriage return
\S	Non-whitespace
[abc]	Character set — matches a, b, or c
[^abc]	Negated set — matches anything except a, b, c
[a-z]	Range — matches any lowercase letter
[a-zA-Z0-9]	Multiple ranges — alphanumeric

Anchors

Pattern	Meaning
^	Start of string (or start of line with m flag)
$	End of string (or end of line with m flag)
\b	Word boundary — between \w and \W
\B	Non-word boundary
\A	Start of string (not affected by m flag) — Python/Ruby
\Z	End of string (not affected by m flag) — Python/Ruby

Quantifiers

Pattern	Meaning
*	Zero or more — greedy
+	One or more — greedy
?	Zero or one (makes preceding element optional)
{n}	Exactly n times
{n,}	n or more times
{n,m}	Between n and m times (inclusive)
*?	Zero or more — lazy (matches as few as possible)
+?	One or more — lazy
??	Zero or one — lazy

Groups & References

Pattern	Meaning
(abc)	Capture group — stores match, referenced as $1 or \1
(?:abc)	Non-capturing group — groups without storing
(?P<name>abc)	Named capture group — referenced as (?P=name)
\1	Backreference to first capture group
(?\|...)	Branch reset group (PCRE only)
a\|b	Alternation — matches a or b

Lookarounds

Pattern	Meaning
(?=abc)	Positive lookahead — followed by abc
(?!abc)	Negative lookahead — not followed by abc
(?<=abc)	Positive lookbehind — preceded by abc
(?<!abc)	Negative lookbehind — not preceded by abc

Flags / Modifiers

Pattern	Meaning
g	Global — find all matches, not just first
i	Case-insensitive
m	Multiline — ^ and $ match start/end of each line
s	Dotall — makes . match newlines too
u	Unicode — enables full Unicode matching
x	Extended — allows whitespace and comments in pattern

Escape Sequences

Pattern	Meaning
\.	Literal dot (escape any metacharacter: * + ? ^ $ { } [ ] \| ( ) \)
\n	Newline
\t	Tab
\r	Carriage return
\0	Null character
\uFFFF	Unicode character by code point
\xFF	Character by hex code

Common Ready-to-Use Patterns

Copy any of these directly into your code or into the regex tester to verify against your own input.

Email (practical)

/^[a-zA-Z0-9._%+\-]+@[a-zA-Z0-9.\-]+\.[a-zA-Z]{2,}$/

Covers the vast majority of valid addresses. See the full guide on email regex for edge cases.

URL (http/https)

/https?:\/\/(www\.)?[-a-zA-Z0-9@:%._+~#=]{1,256}\.[a-zA-Z]{2,6}\b([-a-zA-Z0-9@:%_+.~#?&\/=]*)/

Matches http and https URLs with optional www and path.

IPv4 address

/\b(?:(?:25[0-5]|2[0-4]\d|[01]?\d\d?)\.){3}(?:25[0-5]|2[0-4]\d|[01]?\d\d?)\b/

Validates each octet is 0–255.

Date (YYYY-MM-DD)

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

ISO 8601 date format validation.

Phone (US)

/^\+?1?\s?$?\d{3}$?[\s.\-]?\d{3}[\s.\-]?\d{4}$/

Handles most US phone number formats.

Hex color

/^#?([a-fA-F0-9]{6}|[a-fA-F0-9]{3})$/

Matches 3 or 6 digit hex colors, with or without #.

UUID v4

/^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/i

Validates UUID v4 format.

JWT token

/^[A-Za-z0-9\-_]+\.[A-Za-z0-9\-_]+\.[A-Za-z0-9\-_]*$/

Three base64url segments separated by dots.

Slug (URL-safe)

/^[a-z0-9]+(?:-[a-z0-9]+)*$/

Lowercase alphanumeric with hyphens, no leading/trailing hyphen.

IP + port

/^(\d{1,3}\.){3}\d{1,3}:\d{1,5}$/

Quick match for host:port combinations.

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