RegEx notes

regExNotes.md

RegEx Notes

Regular Expression Basics

expression	description
.	Any character except newline
a	The character a
ab	The string ab
a	b
a*	0 or more a's
\	Escapes a special character

##RegEx escape sequences "­Esc­api­ng" is a way of treating characters which have a special meaning in regular expres­sions literally, rather than as special charac­ters.

expression	description
\	escape the following character
\Q	begin literal sequence
\E	end literal sequence

RegEx quantifiers

expression	description
*	0 or more
+	1 or more
?	0 or 1
{2}	Exactly 2
{2, 5}	Between 2 and 5
2,	2 or more

Note: default is greedy. Append ? for reluctant

RegEx groups & ranges

expression	description
(...)	capturing group
(?:...)	passive non-capturing group
\Y	Match the Y'th captured group
.	Any character except new line (\n)
(a	b)
[abc]	Range (a or b or c)
[^abc]	Not (a or b or c)
[a-q]	Lower case letter from a to q
[A-Q]	Upper case letter from A to Q
[0-7]	Digit from 0 to 7
\x	Group/­sub­pattern number "­x"

Note: Ranges are inclusive

RegEx character classes

expression	description
[ab-d]	One character of: a, b, c, d
[^ab-d]	One character except: a, b, c, d
[\b]	backspace character
\d	One digit
\D	One non-digit
\O	octal digit
\s	One whitespace
\S	One non-white-space
\w	One word character
\W	One non-word character
\x	hexadecimal digit

##RegEx Anchors

expression	description
^	start of string
$	end of string
\b	word boundary
\B	non-word boundary
<	start of word
>	end of word

##RegEx Flags (pattern modifiers)

expression	description
g	global match
i	ignore case
i*	case-sensitive
m	^ and $ match start and end of line
m*	multiple lines
s*	treat string as single line
x*	allow comments and whitespace pattern
e*	evaluate placement
U*	ungreedy pattern

##RegEx special characters

expression	description
\n	Newline
\r	Carriage return
\t	Tab
\0	null character
\YYY	Octal character YYY
\xYY	Hexadecimal character YY
\uYYYY	Hexadecimal character YYYY
\cY	control character

##RegEx replacement

expression	description
$$	inserts $
$&	insert entire match
$`	insert preceding string
$'	insert the following string
$Y	insert Y'th captured group
$n	nth non-pa­ssive group
$2	"­xyz­" in /^(abc­(xy­z))$/
$1	"­xyz­" in /^(?:a­bc)­(xyz)$/
$`	Before matched string
$'	After matched string
$+	Last matched string
$&	Entire matched string

Note: Some regex implem­ent­ations use \ instead of $.

##RegEx Assertions

expression	description
?=	lookahead assertion
?!	negative lookahead
?<=	lookbehind assertion
?!= or ?<!	negative lookbehind
?>	Once-only subexpression
?()	condition, [if then]
?()	(add pipe to end) condition, [if then else]*
?#	comment

*: should be ?()| but wont print to table in markdown

How to replace a all underscored string into spaces

Example: hii_this_is_sudheer
output:hii this is sudheer
#please help me out