🧩 Regular Expressions (Regex) Cheatsheet

A concise, one-page reference for the most common regex tokens and patterns.

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🔤 Literals & Escaping

Pattern	Meaning
abc	Matches literal abc
\.	Escaped dot (matches .)
\\	Escaped backslash
\n	New line
\t	Tab

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🎯 Character Classes

Pattern	Meaning
.	Any character except newline
\d	Digit [0-9]
\D	Non-digit
\w	Word character [A-Za-z0-9_]
\W	Non-word character
\s	Whitespace
\S	Non-whitespace
[abc]	Any of a, b, or c
[^abc]	Not a, b, or c
[a-z]	Range from a to z

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📍 Anchors (Position)

Pattern	Meaning
^	Start of string
$	End of string
\b	Word boundary
\B	Non-word boundary
\A	Start of input (some engines)
\Z	End of input (some engines)

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🔁 Quantifiers

Pattern	Meaning
*	0 or more
+	1 or more
?	0 or 1
{n}	Exactly n times
{n,}	At least n times
{n,m}	Between n and m times
*? +? ??	Lazy (non-greedy) versions

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🔗 Grouping & Alternation

Pattern	Meaning
(abc)	Capturing group
(?:abc)	Non-capturing group
(?<name>abc)	Named capturing group
`a	b`
\1	Backreference to group 1
\k<name>	Backreference to named group

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🔍 Lookarounds (Zero-width Assertions)

Pattern	Meaning
(?=abc)	Positive lookahead
(?!abc)	Negative lookahead
(?<=abc)	Positive lookbehind
(?<!abc)	Negative lookbehind

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🧠 Flags / Modifiers

Flag	Meaning
i	Case-insensitive
g	Global search
m	Multiline (^ and $)
s	Dot matches newline
u	Unicode
x	Ignore whitespace/comments

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🧪 Common Patterns

Use Case	Pattern
Email	^[\w.-]+@[\w.-]+\.\w+$
URL	https?:\/\/[\w.-]+
IPv4	\b(?:\d{1,3}\.){3}\d{1,3}\b
Date (YYYY-MM-DD)	\d{4}-\d{2}-\d{2}
HTML tag	<[^>]+>

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🛠️ Tips

• Prefer specific patterns over .*
• Use lazy quantifiers to avoid overmatching
• Test regex with tools like regex101.com
• Remember: regex engines differ (PCRE, JS, Python)

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📌 **Regex is powerful—but readability matters. Comment complex patterns