Regular Expressions Reference

Overview

A practical and portable regular expression reference for real-world use.
No flag syntax (e.g., (?s), (?m), (?i)) is used to ensure compatibility across OS and tools.
Only safe, environment-independent patterns are included.

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Newline Codes and Platform Differences

OS	Newline Code	Recommended Pattern	Key Point
Windows	\r\n	\r?\n	Universal pattern for CRLF/LF
macOS / Linux	\n	\r?\n	Compatible across all platforms

Key Notes

• Use \r?\n to safely detect or replace newlines on Windows/macOS/Linux.
• To match blocks including newlines, use (?:.|\r|\n) since . alone excludes line breaks.

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Part 1: Basic Syntax Overview

1-1. Character Classes and Ranges

Syntax	Meaning	Example	Match
.	Any single char except newline	a.c / abc	abc
[abc]	One of a, b, or c	bag	a
[^0-9]	Any non-digit	a1	a
[A-Z0-9]	Uppercase letter or digit	X8z	X,8

1-2. Quantifiers

Syntax	Meaning	Example	Match
*	Zero or more	goooogle	goooo
+	One or more	google	oo
?	Zero or one	color colour	Both
{n}	Exactly n times	a{3} / aaa	aaa
{n,}	n or more	a{2,} / aaaa	aaaa
{n,m}	Between n and m	a{2,4} / aaaaa	aaaa

1-3. Anchors

Syntax	Meaning	Example	Match
^abc	Line start	abc\nzabc	abc (line 1)
abc$	Line end	zabc\nabc	Both line ends
\bword\b	Word boundary	word words	word
\Bing	Non-boundary	ringing	second ing

1-4. Shortcuts

Syntax	Meaning	Example	Match
\d	Digit [0-9]	ver2.10	2,10
\D	Non-digit	a1	a
\w	Alphanumeric + _	a_b-1	a_b
\W	Non-alphanumeric	a#	#
\s	Whitespace (tab/newline etc.)	a b	space
\S	Non-whitespace	a b	a,b
\t	Tab	a\tb	a[TAB]b

1-5. Escapes and Special Characters

Syntax	Meaning	Example	Match
\.	Literal dot	a.c	a.c
\*	Asterisk	a*b	a*b
\+	Plus sign	a+b	a+b
\?	Question mark	what?	what?
\( \)	Parentheses	(test)	(test)
|	Pipe	`a	b`
\\	Backslash	C:\\path	\\
\^	Caret	^abc	^abc
\$	Dollar	total$	$
\[ \]	Brackets	[abc]	[abc]
\{ \}	Braces	{a,b}	{a,b}

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Part 2: Grouping, Alternation, Lookaround

Syntax	Purpose	Example	Match
(abc)+	Repeat as group	abcabcx	abcabc
`(?:jpg	png)`	Non-capturing OR	file.png
`foo	bar`	OR condition	bar
\d+(?=円)	Numbers before “円”	合計100円	100
^(?!.*error).*	Line not containing “error”	ok\nerror	ok
(?<=￥)\d+	Number after “￥”	￥300	300
(?<!Mr\.)\s[A-Z]	Uppercase not after “Mr.”	Ms. Alice	A

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Part 3: Matching Multi-line Text

Use Case	Pattern	Example	Match
HTML block	` (?:.	\r	\n)*?`
Log entry	`^[\d{4}-\d{2}-\d{2} [\d:]+](?:.	\r	\n)*?(?=^[\d{4}-\d{2}-\d{2}
Markdown code block	“ ```(?:.	\r	\n)*?``` “
Comment (/*…*/)	`/*(?:.	\r	\n)*?*/`

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Part 4: Pattern Library (Filtering / Extraction)

Purpose	Pattern	Example	Match
Digits only	^\d+$	123	123
Alphanumeric	^[A-Za-z0-9]+$	user01	user01
Email (simple)	^[\w.-]+@[\w.-]+\.[A-Za-z]{2,}$	a@b.com	a@b.com
URL	https?://[\w.-]+\.[A-Za-z]{2,}(/[\w./?=&%-]*)?	https://ex.com/a	https://ex.com/a
ISO date	\d{4}-\d{2}-\d{2}	2025-10-30	2025-10-30
Intl. phone	\+\d{1,3}[\s-]?\d{1,14}	+81 90 1234 5678	full
Strong password	^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).{8,}$	Abcd1234	full
HTML comment	`<!—(?:.	\r	\n)*?—>`
Lines without “error”	^(?!.*error).*	ok\nerror	ok

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Part 5: Replacement Examples

Task	Pattern	Replace	Input	Output
Reverse words	(\w+)\s+(\w+)	${2} ${1}	John Doe	Doe John
Change delimiter	;	,	a;b;c	a,b,c
Normalize spaces	\s{2,}		a b	a b
Remove HTML tags	<[^>]+>	“	<p>a</p>	a
Trim spaces	`^\s+	\s+$`	“	a
Newline → space	\r?\n		a\nb	a b
Remove end comment	//.*$	“	x=1;//note	x=1;
Unify date format	(\d{4})/(\d{2})/(\d{2})	${1}-${2}-${3}	2025/10/30	2025-10-30
Compress duplicate lines	^(.*)(\r?\n\1)+$	${1}	repeated lines	single line

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Summary

Regular expressions are a cross-platform and multilingual tool for text processing, scripting, log parsing, and data cleanup.
Using flag-independent, portable patterns ensures stability across diverse environments and editors.