Regex Cheatsheet

Character Classes

Pattern Matches Example

Pattern	Matches	Example
`.`	Any character except newline	`a.c` matches "abc", "a1c"
`\d`	Digit [0-9]	`\d{3}` matches "123"
`\D`	Non-digit	`\D+` matches "abc"
`\w`	Word char [a-zA-Z0-9_]	`\w+` matches "var_1"
`\W`	Non-word char	`\W` matches "@", " "
`\s`	Whitespace [ \t\n\r\f]	`\s+` matches spaces/tabs
`\S`	Non-whitespace	`\S+` matches "word"
`[abc]`	Character set — a, b, or c	`[aeiou]` matches vowels
`[^abc]`	Negated set — not a, b, or c	`[^0-9]` matches non-digits
`[a-z]`	Range	`[A-Fa-f0-9]` matches hex chars

.

Any character except newline

a.c matches "abc", "a1c"

\d

Digit [0-9]

\d{3} matches "123"

\D

Non-digit

\D+ matches "abc"

\w

Word char [a-zA-Z0-9_]

\w+ matches "var_1"

\W

Non-word char

\W matches "@", " "

\s

Whitespace [ \t\n\r\f]

\s+ matches spaces/tabs

\S

Non-whitespace

\S+ matches "word"

[abc]

Character set — a, b, or c

[aeiou] matches vowels

[^abc]

Negated set — not a, b, or c

[^0-9] matches non-digits

[a-z]

Range

[A-Fa-f0-9] matches hex chars

Anchors

Pattern Matches Example

Pattern	Matches	Example
`^`	Start of line	`^Error` matches "Error: …"
`$`	End of line	`\.conf$` matches "sshd.conf"
`\b`	Word boundary	`\bcat\b` matches "cat" not "concatenate"
`\B`	Non-word boundary	`\Bcat` matches "concatenate"

^

Start of line

^Error matches "Error: …"

$

End of line

\.conf$ matches "sshd.conf"

\b

Word boundary

\bcat\b matches "cat" not "concatenate"

\B

Non-word boundary

\Bcat matches "concatenate"

Quantifiers

Pattern Matches Notes

Pattern	Matches	Notes
`*`	0 or more (greedy)	`a*` matches "", "a", "aaa"
`+`	1 or more (greedy)	`a+` matches "a", "aaa" not ""
`?`	0 or 1 (optional)	`colou?r` matches "color", "colour"
`{n}`	Exactly n	`\d{4}` matches "2026"
`{n,}`	n or more	`\d{2,}` matches "12", "123"
`{n,m}`	Between n and m	`\d{1,3}` matches "1", "12", "123"
`*?`	0 or more (lazy)	Matches as few as possible
`+?`	1 or more (lazy)	Matches as few as possible

*

0 or more (greedy)

a* matches "", "a", "aaa"

+

1 or more (greedy)

a+ matches "a", "aaa" not ""

?

0 or 1 (optional)

colou?r matches "color", "colour"

{n}

Exactly n

\d{4} matches "2026"

{n,}

n or more

\d{2,} matches "12", "123"

{n,m}

Between n and m

\d{1,3} matches "1", "12", "123"

*?

0 or more (lazy)

Matches as few as possible

+?

1 or more (lazy)

Matches as few as possible

Groups and Alternation

Pattern Purpose Example

Pattern	Purpose	Example
`(abc)`	Capture group	`(error\|warn)` captures match
`(?:abc)`	Non-capturing group	`(?:http\|https)://` groups without capturing
`\1`	Backreference to group 1	`(\w+)\s+\1` matches "the the"
`a\|b`	Alternation (or)	`cat\|dog` matches "cat" or "dog"

(abc)

Capture group

(error|warn) captures match

(?:abc)

Non-capturing group

(?:http|https):// groups without capturing

\1

Backreference to group 1

(\w+)\s+\1 matches "the the"

a|b

Alternation (or)

cat|dog matches "cat" or "dog"

Lookaround (PCRE)

Pattern Purpose Example

Pattern	Purpose	Example
`(?=…)`	Lookahead (followed by)	`\d+(?=px)` matches "12" in "12px"
`(?!…)`	Negative lookahead	`\d+(?!px)` matches "12" in "12em"
`(?⇐…)`	Lookbehind (preceded by)	`(?⇐\$)\d+` matches "50" in "$50"
`(?<!…)`	Negative lookbehind	`(?<!\$)\d+` matches "50" in "EUR50"

(?=…)

Lookahead (followed by)

\d+(?=px) matches "12" in "12px"

(?!…)

Negative lookahead

\d+(?!px) matches "12" in "12em"

(?⇐…)

Lookbehind (preceded by)

(?⇐\$)\d+ matches "50" in "$50"

(?<!…)

Negative lookbehind

(?<!\$)\d+ matches "50" in "EUR50"

Common Patterns

IP address (basic)

\b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\b

Email (simple)

[\w.+-]+@[\w-]+\.[\w.]+

MAC address

([0-9A-Fa-f]{2}[:-]){5}[0-9A-Fa-f]{2}

Log timestamp (ISO 8601)

\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}

CLI Usage

grep with PCRE

grep -P '(?<=error:\s)\w+' /var/log/syslog

sed capture groups

echo "2026-04-10" | sed 's/\([0-9]\{4\}\)-\([0-9]\{2\}\)-\([0-9]\{2\}\)/\2\/\3\/\1/'

awk regex match

awk '/^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+/ {print $1}' access.log