Regex.md

Regular expressions (regex) are sequences of characters that define search patterns. They are widely used in text processing, searching, and validation.

Reference: GeekHour youtube video Practice: Regexr regex101

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Table of Contents

1. Basic and Quantifier Syntax

2. Greedy vs. Non-Greedy Matching

3. Character Classes

4. Grouping and Backreferences

5. Flags

6. Common Use Cases

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1. Basic and Quantifier Syntax

Symbol	Description	Example	Matches
.	Matches any single character (except newline)	c.t	cat, cut, cot
^	Matches the start of a line	^Hello	Matches "Hello" only at the beginning of a line
$	Matches the end of a line	world$	Matches "world" only at the end of a line
*	Matches 0 or more occurrences of the previous character	ba*	b, ba, baa, baaa
+	Matches 1 or more occurrences of the previous character	ba+	ba, baa, baaa
?	Matches 0 or 1 occurrence of the previous character	ba?	b, ba
{n}	Matches exactly n occurrences	a{3}	aaa
{n,}	Matches n or more occurrences	a{2,}	aa, aaa, aaaa
{n,m}	Matches between n and m occurrences	a{2,4}	aa, aaa, aaaa
|	OR operator (matches either pattern)	cat|dog	cat, dog
()	Groups expressions	(ab)+	ab, abab, ababab

2. Greedy vs. Non-Greedy Matching

Type	Symbol	Description	Example	Matches
Greedy	.*	Matches as much text as possible	a.*b on axxxxb	axxxxb
Non-Greedy	.*?	Matches the shortest possible text	a.*?b on axxxxb	axb

3. Character Classes

Character Class	Description	Example	Matches
[abc]	Matches any one of the specified characters	[abc]	a, b, c
[^abc]	Matches any character except those specified	[^abc]	Any character except a, b, c
[a-z]	Matches any lowercase letter	[a-z]	a, b, ..., z
[A-Z]	Matches any uppercase letter	[A-Z]	A, B, ..., Z
[0-9]	Matches any digit	[0-9]	0, 1, ..., 9
[a-zA-Z0-9]	Matches any alphanumeric character	[a-zA-Z0-9]	a, B, 3
.	Matches any character except newline	.	a, 1, @

4. Grouping and Backreferences

Symbol	Description	Example	Matches
(...)	Capturing group	(abc)	Matches abc and stores it in memory
(?:...)	Non-capturing group	(?:abc)	Matches abc but does not store it
\1, \2, ... ($1, $2, ...)	Backreference to captured group	(\w+) \1	Matches hello hello, abc abc

Example Usage of Backreferences

(\d{3})-(\d{3})-(\d{4})

• Matches phone numbers formatted as 123-456-7890

• \1, \2, and \3 refer to the first, second, and third captured groups respectively.

5. Flags

Flag	Description
i	Case-insensitive match
g	Global match (find all matches)
m	Multi-line mode (^ and $ match start and end of lines)
s	Dotall mode (. matches newline)
x	Ignore whitespace for readability

6. Common Use Cases

Matching Email Addresses

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

Matching Phone Numbers (US Format)

\(\d{3}\) \d{3}-\d{4}

Extracting URLs

https?://[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}(/[a-zA-Z0-9._%+-]*)*

Matching Hex Colors

#([a-fA-F0-9]{6}|[a-fA-F0-9]{3})

Matching Dates (YYYY-MM-DD)

\d{4}-\d{2}-\d{2}

Summary

Regular expressions are a powerful tool for searching, matching, and manipulating text. By mastering regex patterns, you can efficiently handle complex text-processing tasks in various programming languages and command-line tools.