Regex Cheat Sheet

Q: What is a regular expression (regex)?

A regular expression is a sequence of characters that defines a search pattern. It is used for string searching, matching, and manipulation. Regex is supported natively in JavaScript, Python, Java, PHP, Go, and most modern programming languages.

Q: What is the difference between greedy and lazy quantifiers?

Greedy quantifiers (*, +, {n,m}) match as many characters as possible. Lazy quantifiers (*?, +?, {n,m}?) match as few characters as possible. For example, on ' bold ' greedy matches the entire string, while matches ' ' and ' ' separately.

Q: Are regex patterns the same in all languages?

Core syntax (character classes, quantifiers, anchors) is consistent across most languages. However, features like lookbehind, named groups, and specific flags vary by engine. JavaScript uses PCRE-like syntax without lookbehind in older versions; Python, PHP, and Java support full PCRE. Always test in your target language.

Q: Will every reference pattern work in every regex engine?

No. The examples illustrate syntax, while engines differ in flags, Unicode rules, lookbehind support, and named groups. Test a copied pattern in the linked JavaScript tester and in your destination runtime. Include matching, nonmatching, and boundary cases; a pattern that works for one short example may still be expensive on longer input.

Try in Regex Tester →

Character Classes

Syntax	Description	Example	Matches
`.`	Any character except newline	`h.t`	hat, hit, hot
`\d`	Digit [0-9]	`\d+`	123, 42, 7
`\D`	Non-digit [^0-9]	`\D+`	abc, foo, !
`\w`	Word char [a-zA-Z0-9_]	`\w+`	hello, foo_bar
`\W`	Non-word character	`\W+`	!@#, spaces
`\s`	Whitespace (space, tab, newline)	`a\sb`	'a b', 'a\tb'
`\S`	Non-whitespace	`\S+`	hello, 123
`[abc]`	One of a, b, or c	`[aeiou]`	'a', 'e', 'i'
`[^abc]`	Not a, b, or c	`[^aeiou]`	'b', 'c', '1'
`[a-z]`	Character in range a–z	`[a-f]`	'a', 'b', 'f'
`[a-zA-Z]`	Letter (any case)	`[a-zA-Z]+`	hello, World
`\p{L}`	Any Unicode letter (u flag)	`\p{L}+`	café, 你好

Anchors & Boundaries

Syntax	Description	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	`\bcat\b`	'cat' but not 'catfish'
`\B`	Non-word boundary	`\Bcat`	'catfish' (not standalone 'cat')
`\A`	Start of string (Python/Java)	`\AHello`	Hello world
`\Z`	End of string (Python/Java)	`world\Z`	Hello world

Quantifiers

Syntax	Description	Example	Matches
`*`	0 or more (greedy)	`a*`	'', 'a', 'aaa'
`+`	1 or more (greedy)	`a+`	'a', 'aaa'
`?`	0 or 1 (optional)	`colou?r`	color, colour
`{n}`	Exactly n times	`\d{4}`	'2024', '1999'
`{n,}`	n or more times	`\d{2,}`	'12', '1234'
`{n,m}`	Between n and m times	`\d{2,4}`	'12', '123', '1234'
`*?`	0 or more (lazy)	`<.*?>`	'<b>' from '<b>text</b>'
`+?`	1 or more (lazy)	`<.+?>`	'<b>' from '<b>text</b>'
`??`	0 or 1 (lazy)	`a??b`	'b' or 'ab'

Groups & References

Syntax	Description	Example	Matches
`(abc)`	Capturing group	`(\d+)-(\d+)`	Captures both numbers in '2024-01'
`(?:abc)`	Non-capturing group	`(?:foo\|bar)baz`	'foobaz', 'barbaz'
`(?<name>abc)`	Named capturing group	`(?<year>\d{4})`	Named capture of year
`\1`	Backreference to group 1	`(\w+)\s\1`	'the the', 'word word'
`\k<name>`	Named backreference	`(?<tag>\w+).*\k<tag>`	Matching HTML tags
`(?\|...)`	Branch reset group (PCRE)	`(?\|(a)\|(b))`	Both captured in group 1

Lookahead & Lookbehind

Syntax	Description	Example	Matches
`(?=abc)`	Positive lookahead — must be followed by abc	`\d+(?= dollars)`	'100' in '100 dollars'
`(?!abc)`	Negative lookahead — must NOT be followed by abc	`\d+(?! dollars)`	'100' in '100 euros'
`(?<=abc)`	Positive lookbehind — must be preceded by abc	`(?<=\$)\d+`	'100' in '$100'
`(?<!abc)`	Negative lookbehind — must NOT be preceded by abc	`(?<!\$)\d+`	'100' in '100 items' but not '$100'

Flags (Modifiers)

Flag	Name	Description
`g`	Global	Find all matches, not just the first
`i`	Case-insensitive	Match regardless of case (A = a)
`m`	Multiline	^ and $ match start/end of each line
`s`	Dotall	. matches newline characters too
`u`	Unicode	Enable full Unicode support (\p{} classes)
`y`	Sticky	Match only at lastIndex position (JS)
`x`	Extended	Allow whitespace + comments in pattern (Python/PHP)

Common Patterns

Pattern	Regex	Test
Email	`[a-zA-Z0-9._%+\-]+@[a-zA-Z0-9.\-]+\.[a-zA-Z]{2,}`	Try in Regex Tester →
URL	`https?:\/\/[\w\-]+(\.[\w\-]+)+[\w\-._~:/?#[\]@!$&'()+,;=%]`	Try in Regex Tester →
IPv4 Address	`\b(?:(?:25[0-5]\|2[0-4]\d\|[01]?\d\d?)\.){3}(?:25[0-5]\|2[0-4]\d\|[01]?\d\d?)\b`	Try in Regex Tester →
Phone (US)	`\+?1?[\s.-]?$?\d{3}$?[\s.-]?\d{3}[\s.-]?\d{4}`	Try in Regex Tester →
Date (YYYY-MM-DD)	`\d{4}-(?:0[1-9]\|1[0-2])-(?:0[1-9]\|[12]\d\|3[01])`	Try in Regex Tester →
Time (HH:MM)	`(?:[01]\d\|2[0-3]):[0-5]\d`	Try in Regex Tester →
UUID	`[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}`	Try in Regex Tester →
Hex Color	`#(?:[0-9a-fA-F]{3}){1,2}\b`	Try in Regex Tester →
Slug	`[a-z0-9]+(?:-[a-z0-9]+)*`	Try in Regex Tester →
ZIP Code (US)	`\d{5}(?:-\d{4})?`	Try in Regex Tester →
Credit Card	`(?:4[0-9]{12}(?:[0-9]{3})?\|5[1-5][0-9]{14}\|3[47][0-9]{13})`	Try in Regex Tester →
HTML Tag	`<([a-zA-Z][a-zA-Z0-9])(?:\s[^>])?\/?>.*?<\/\1>`	Try in Regex Tester →
Markdown Bold	`\\([^]+)\\*`	Try in Regex Tester →
JWT Token	`[A-Za-z0-9-_]+\.[A-Za-z0-9-_]+\.[A-Za-z0-9-_]+`	Try in Regex Tester →
Semver	`\bv?(?:0\|[1-9]\d)\.(?:0\|[1-9]\d)\.(?:0\|[1-9]\d*)(?:-[\w.]+)?\b`	Try in Regex Tester →
GitHub Username	`(?<=github\.com\/)([a-zA-Z0-9](?:[a-zA-Z0-9-]{0,37}[a-zA-Z0-9])?)`	Try in Regex Tester →
Positive Integer	`^[1-9]\d*$`	Try in Regex Tester →
Float / Decimal	`-?\d+(?:\.\d+)?`	Try in Regex Tester →
Blank Lines	`^\s*$`	Try in Regex Tester →
Duplicate Words	`\b(\w+)\s+\1\b`	Try in Regex Tester →

Regex Cheat Sheet: Patterns Every Developer Needs

Hex Color Codes

^#?([0-9A-Fa-f]{3}|[0-9A-Fa-f]{6})$

Convert between color formats with our Color Converter.

Regex Performance Tips

Avoid catastrophic backtracking — Nested quantifiers like (a+)+ cause exponential time
Use non-capturing groups when you do not need the value
Test with edge cases — Use our Regex Tester before deploying

Frequently Asked Questions

What is the difference between `*` and `+`?

* matches zero or more occurrences, + requires at least one. So ab*c matches "ac" but ab+c does not.

How do I make regex case-insensitive?

Use the i flag: /pattern/i in JavaScript, re.IGNORECASE in Python.

UUID Validation

^[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}$

This pattern checks the text shape only, not the UUID version or variant.

Frequently Asked Questions

What is a regular expression (regex)?

A regular expression is a sequence of characters that defines a search pattern. It is used for string searching, matching, and manipulation. Regex is supported natively in JavaScript, Python, Java, PHP, Go, and most modern programming languages.

What is the difference between greedy and lazy quantifiers?

Greedy quantifiers (*, +, {n,m}) match as many characters as possible. Lazy quantifiers (*?, +?, {n,m}?) match as few characters as possible. For example, <.+> on '<b>bold</b>' greedy matches the entire string, while <.+?> matches '<b>' and '</b>' separately.

What are lookaheads and lookbehinds?

Lookaheads (?=...) and lookbehinds (?<=...) are zero-width assertions — they check for a pattern without consuming characters. Positive lookahead (?=foo) asserts that 'foo' follows. Negative lookahead (?!foo) asserts that 'foo' does NOT follow. They are extremely useful for complex validations.

Are regex patterns the same in all languages?

Core syntax (character classes, quantifiers, anchors) is consistent across most languages. However, features like lookbehind, named groups, and specific flags vary by engine. JavaScript uses PCRE-like syntax without lookbehind in older versions; Python, PHP, and Java support full PCRE. Always test in your target language.

Will every reference pattern work in every regex engine?

No. The examples illustrate syntax, while engines differ in flags, Unicode rules, lookbehind support, and named groups. Test a copied pattern in the linked JavaScript tester and in your destination runtime. Include matching, nonmatching, and boundary cases; a pattern that works for one short example may still be expensive on longer input.

Regex Tester Online: Test Regular Expressions HTTP Status Codes — Complete Reference MIME Types List: File Extensions and Content-Type Keyboard Shortcuts Reference ASCII Table — Complete Character Codes Reference Unix Timestamp & Epoch Reference – Formats & Examples

Regex Cheat Sheet

Try in Regex Tester →

Character Classes

Syntax	Description	Example	Matches
`.`	Any character except newline	`h.t`	hat, hit, hot
`\d`	Digit [0-9]	`\d+`	123, 42, 7
`\D`	Non-digit [^0-9]	`\D+`	abc, foo, !
`\w`	Word char [a-zA-Z0-9_]	`\w+`	hello, foo_bar
`\W`	Non-word character	`\W+`	!@#, spaces
`\s`	Whitespace (space, tab, newline)	`a\sb`	'a b', 'a\tb'
`\S`	Non-whitespace	`\S+`	hello, 123
`[abc]`	One of a, b, or c	`[aeiou]`	'a', 'e', 'i'
`[^abc]`	Not a, b, or c	`[^aeiou]`	'b', 'c', '1'
`[a-z]`	Character in range a–z	`[a-f]`	'a', 'b', 'f'
`[a-zA-Z]`	Letter (any case)	`[a-zA-Z]+`	hello, World
`\p{L}`	Any Unicode letter (u flag)	`\p{L}+`	café, 你好

Anchors & Boundaries

Syntax	Description	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	`\bcat\b`	'cat' but not 'catfish'
`\B`	Non-word boundary	`\Bcat`	'catfish' (not standalone 'cat')
`\A`	Start of string (Python/Java)	`\AHello`	Hello world
`\Z`	End of string (Python/Java)	`world\Z`	Hello world

Quantifiers

Syntax	Description	Example	Matches
`*`	0 or more (greedy)	`a*`	'', 'a', 'aaa'
`+`	1 or more (greedy)	`a+`	'a', 'aaa'
`?`	0 or 1 (optional)	`colou?r`	color, colour
`{n}`	Exactly n times	`\d{4}`	'2024', '1999'
`{n,}`	n or more times	`\d{2,}`	'12', '1234'
`{n,m}`	Between n and m times	`\d{2,4}`	'12', '123', '1234'
`*?`	0 or more (lazy)	`<.*?>`	'<b>' from '<b>text</b>'
`+?`	1 or more (lazy)	`<.+?>`	'<b>' from '<b>text</b>'
`??`	0 or 1 (lazy)	`a??b`	'b' or 'ab'

Groups & References

Syntax	Description	Example	Matches
`(abc)`	Capturing group	`(\d+)-(\d+)`	Captures both numbers in '2024-01'
`(?:abc)`	Non-capturing group	`(?:foo\|bar)baz`	'foobaz', 'barbaz'
`(?<name>abc)`	Named capturing group	`(?<year>\d{4})`	Named capture of year
`\1`	Backreference to group 1	`(\w+)\s\1`	'the the', 'word word'
`\k<name>`	Named backreference	`(?<tag>\w+).*\k<tag>`	Matching HTML tags
`(?\|...)`	Branch reset group (PCRE)	`(?\|(a)\|(b))`	Both captured in group 1

Lookahead & Lookbehind

Syntax	Description	Example	Matches
`(?=abc)`	Positive lookahead — must be followed by abc	`\d+(?= dollars)`	'100' in '100 dollars'
`(?!abc)`	Negative lookahead — must NOT be followed by abc	`\d+(?! dollars)`	'100' in '100 euros'
`(?<=abc)`	Positive lookbehind — must be preceded by abc	`(?<=\$)\d+`	'100' in '$100'
`(?<!abc)`	Negative lookbehind — must NOT be preceded by abc	`(?<!\$)\d+`	'100' in '100 items' but not '$100'

Flags (Modifiers)

Flag	Name	Description
`g`	Global	Find all matches, not just the first
`i`	Case-insensitive	Match regardless of case (A = a)
`m`	Multiline	^ and $ match start/end of each line
`s`	Dotall	. matches newline characters too
`u`	Unicode	Enable full Unicode support (\p{} classes)
`y`	Sticky	Match only at lastIndex position (JS)
`x`	Extended	Allow whitespace + comments in pattern (Python/PHP)

Common Patterns

Pattern	Regex	Test
Email	`[a-zA-Z0-9._%+\-]+@[a-zA-Z0-9.\-]+\.[a-zA-Z]{2,}`	Try in Regex Tester →
URL	`https?:\/\/[\w\-]+(\.[\w\-]+)+[\w\-._~:/?#[\]@!$&'()+,;=%]`	Try in Regex Tester →
IPv4 Address	`\b(?:(?:25[0-5]\|2[0-4]\d\|[01]?\d\d?)\.){3}(?:25[0-5]\|2[0-4]\d\|[01]?\d\d?)\b`	Try in Regex Tester →
Phone (US)	`\+?1?[\s.-]?$?\d{3}$?[\s.-]?\d{3}[\s.-]?\d{4}`	Try in Regex Tester →
Date (YYYY-MM-DD)	`\d{4}-(?:0[1-9]\|1[0-2])-(?:0[1-9]\|[12]\d\|3[01])`	Try in Regex Tester →
Time (HH:MM)	`(?:[01]\d\|2[0-3]):[0-5]\d`	Try in Regex Tester →
UUID	`[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}`	Try in Regex Tester →
Hex Color	`#(?:[0-9a-fA-F]{3}){1,2}\b`	Try in Regex Tester →
Slug	`[a-z0-9]+(?:-[a-z0-9]+)*`	Try in Regex Tester →
ZIP Code (US)	`\d{5}(?:-\d{4})?`	Try in Regex Tester →
Credit Card	`(?:4[0-9]{12}(?:[0-9]{3})?\|5[1-5][0-9]{14}\|3[47][0-9]{13})`	Try in Regex Tester →
HTML Tag	`<([a-zA-Z][a-zA-Z0-9])(?:\s[^>])?\/?>.*?<\/\1>`	Try in Regex Tester →
Markdown Bold	`\\([^]+)\\*`	Try in Regex Tester →
JWT Token	`[A-Za-z0-9-_]+\.[A-Za-z0-9-_]+\.[A-Za-z0-9-_]+`	Try in Regex Tester →
Semver	`\bv?(?:0\|[1-9]\d)\.(?:0\|[1-9]\d)\.(?:0\|[1-9]\d*)(?:-[\w.]+)?\b`	Try in Regex Tester →
GitHub Username	`(?<=github\.com\/)([a-zA-Z0-9](?:[a-zA-Z0-9-]{0,37}[a-zA-Z0-9])?)`	Try in Regex Tester →
Positive Integer	`^[1-9]\d*$`	Try in Regex Tester →
Float / Decimal	`-?\d+(?:\.\d+)?`	Try in Regex Tester →
Blank Lines	`^\s*$`	Try in Regex Tester →
Duplicate Words	`\b(\w+)\s+\1\b`	Try in Regex Tester →

Regex Cheat Sheet: Patterns Every Developer Needs

Hex Color Codes

^#?([0-9A-Fa-f]{3}|[0-9A-Fa-f]{6})$

Convert between color formats with our Color Converter.

Regex Performance Tips

Avoid catastrophic backtracking — Nested quantifiers like (a+)+ cause exponential time
Use non-capturing groups when you do not need the value
Test with edge cases — Use our Regex Tester before deploying

Frequently Asked Questions

What is the difference between `*` and `+`?

* matches zero or more occurrences, + requires at least one. So ab*c matches "ac" but ab+c does not.

How do I make regex case-insensitive?

Use the i flag: /pattern/i in JavaScript, re.IGNORECASE in Python.

UUID Validation

^[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}$

Regex Cheat Sheet

Character Classes

Anchors & Boundaries

Quantifiers

Groups & References

Lookahead & Lookbehind

Flags (Modifiers)

Common Patterns

Regex Cheat Sheet: Patterns Every Developer Needs

Hex Color Codes

Regex Performance Tips

Frequently Asked Questions

What is the difference between * and +?

How do I make regex case-insensitive?

UUID Validation

Frequently Asked Questions

What is a regular expression (regex)?

What is the difference between greedy and lazy quantifiers?

What are lookaheads and lookbehinds?

Are regex patterns the same in all languages?

Will every reference pattern work in every regex engine?

Related Tools

Regex Cheat Sheet

Character Classes

Anchors & Boundaries

Quantifiers

Groups & References

Lookahead & Lookbehind

Flags (Modifiers)

Common Patterns

Regex Cheat Sheet: Patterns Every Developer Needs

Hex Color Codes

Regex Performance Tips

Frequently Asked Questions

What is the difference between * and +?

How do I make regex case-insensitive?

UUID Validation

Frequently Asked Questions

What is a regular expression (regex)?

What is the difference between greedy and lazy quantifiers?

What are lookaheads and lookbehinds?

Are regex patterns the same in all languages?

Will every reference pattern work in every regex engine?

Related Tools

What is the difference between `*` and `+`?

What is the difference between `*` and `+`?