Regex Cheatsheet — Regular Expression Syntax Reference
Complete regular expression reference with metacharacters, quantifiers, anchors, groups, lookaheads, flags, and practical examples for pattern matching.
Regular expressions (regex) are patterns used to match character combinations in strings. They appear in virtually every programming language and many command-line tools. This reference covers the full regex syntax you need for day-to-day development.
Metacharacters
These special characters have meaning beyond their literal value.
| Character | Meaning | Example | Matches |
|---|---|---|---|
. | Any character except newline | a.c | abc, a1c, a-c |
\d | Any digit (0-9) | \d{3} | 123, 456 |
\D | Any non-digit | \D+ | abc, --- |
\w | Word character (a-z, A-Z, 0-9, _) | \w+ | hello_42 |
\W | Non-word character | \W | @, #, |
\s | Whitespace (space, tab, newline) | \s+ | , \t\n |
\S | Non-whitespace | \S+ | hello |
\b | Word boundary | \bcat\b | cat in “the cat sat” |
\B | Non-word boundary | \Bcat\B | cat in “concatenate” |
\\ | Escape special character | \. | literal . |
Quantifiers
Quantifiers control how many times a pattern element repeats.
| Quantifier | Meaning | Example | Matches |
|---|---|---|---|
* | 0 or more | ab*c | ac, abc, abbc |
+ | 1 or more | ab+c | abc, abbc (not ac) |
? | 0 or 1 | colou?r | color, colour |
{n} | Exactly n | \d{4} | 2026 |
{n,} | n or more | \d{2,} | 42, 123, 9999 |
{n,m} | Between n and m | \d{2,4} | 42, 123, 9999 |
Lazy (non-greedy) versions: Add ? after any quantifier to match as few characters as possible.
| Greedy | Lazy | Behavior |
|---|---|---|
.* | .*? | Match as little as possible |
.+ | .+? | Match 1+, preferring fewer |
.{2,5} | .{2,5}? | Match 2-5, preferring 2 |
Anchors
Anchors match positions, not characters.
| Anchor | Meaning | Example | Matches |
|---|---|---|---|
^ | Start of string (or line with m flag) | ^Hello | Hello world |
$ | End of string (or line with m flag) | world$ | Hello world |
\b | Word boundary | \bword\b | whole word only |
\A | Start of string (never line) | \AStart | only at absolute start |
\Z | End of string (never line) | end\Z | only at absolute end |
Character Classes
| Syntax | Meaning | Example | Matches |
|---|---|---|---|
[abc] | Any of a, b, or c | [aeiou] | any vowel |
[^abc] | Not a, b, or c | [^0-9] | any non-digit |
[a-z] | Range: a through z | [A-Za-z] | any letter |
[a-zA-Z0-9] | Alphanumeric | [a-zA-Z0-9_] | same as \w |
[\s\S] | Any character including newline | [\s\S]* | everything |
Groups and Capturing
| Syntax | Meaning | Example |
|---|---|---|
(abc) | Capturing group | (foo)bar captures foo |
(?:abc) | Non-capturing group | (?:foo)bar groups without capture |
(?<name>abc) | Named capturing group | (?<year>\d{4}) |
\1 | Backreference to group 1 | (a)\1 matches aa |
\k<name> | Named backreference | \k<year> |
(a|b) | Alternation (OR) | (cat|dog) matches either |
Lookahead and Lookbehind
These match a position based on what comes before or after, without consuming characters.
| Syntax | Name | Example | Matches |
|---|---|---|---|
(?=abc) | Positive lookahead | \d(?=px) | 5 in 5px |
(?!abc) | Negative lookahead | \d(?!px) | 5 in 5em |
(?<=abc) | Positive lookbehind | (?<=\$)\d+ | 100 in $100 |
(?<!abc) | Negative lookbehind | (?<!\$)\d+ | 100 in €100 |
Flags / Modifiers
Flags change how the regex engine processes the pattern.
| Flag | Name | Effect |
|---|---|---|
g | Global | Find all matches, not just the first |
i | Case-insensitive | A matches a |
m | Multiline | ^ and $ match line starts/ends |
s | Dotall / Single-line | . matches newline characters |
u | Unicode | Enable full Unicode matching |
x | Extended | Ignore whitespace, allow comments |
y | Sticky | Match only at lastIndex position |
Common Patterns
These battle-tested patterns cover frequent validation tasks.
# Email (simplified)
^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$
# URL
https?:\/\/(www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b([-a-zA-Z0-9()@:%_\+.~#?&//=]*)
# IPv4 Address
\b(?:\d{1,3}\.){3}\d{1,3}\b
# Date (YYYY-MM-DD)
\d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])
# Hex color
#?([a-fA-F0-9]{6}|[a-fA-F0-9]{3})
# Phone (US)
\(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}
# Strong password (8+ chars, upper, lower, digit, special)
^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$
# HTML tag
<\/?[\w\s]*>|<.+[\W]>
# Whitespace trimming
^\s+|\s+$POSIX Character Classes
Used in tools like grep, sed, and awk.
| Class | Equivalent | Meaning |
|---|---|---|
[:alpha:] | [a-zA-Z] | Letters |
[:digit:] | [0-9] | Digits |
[:alnum:] | [a-zA-Z0-9] | Alphanumeric |
[:space:] | [\s] | Whitespace |
[:upper:] | [A-Z] | Uppercase letters |
[:lower:] | [a-z] | Lowercase letters |
[:punct:] | Punctuation characters | |
[:print:] | Printable characters |
Test your patterns in real time with the Regex Tester or generate patterns automatically with the Regex Generator.
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