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Escaped characters

Metacharacter	Description
\. \* \\	The backslash is used to escape a special character

Control characters

Metacharacter	Description
\t	Find a tab character
\v	Find a vertical tab character
\n	Find a new line character
\r	Find a carriage return character

Groups & Lookaround

Metacharacter	Description
[abc]	any of a, b, or c
[^abc]	not a, b, or c
[a-g]	the character between a & g
[A-Z]	Any character from uppercase A to uppercase Z
[a-z]	Any character from lowercase a to lowercase z
[A-z]	Any character from uppercase A to lowercase z
[0-9]	any numbers or span of numbers from 0 to 9
(abc)	capture group
\1	a backreference to group #1
(?:abc)	non-capturing group
(?=abc)	positive lookahead
(?!abc)	negative lookahead

Opening and closing square bracket matches a range of characters

Opening and closing parenthesis used to group characters

A(?=B) → look for A, but match only if followed by B.

A(?!B) → look for A, but match only if not followed by B.

let text = `I have 2 notes of 100₹`

let pattern = /\d+(?=₹)/gm;
text.match(pattern)

// Output
['100']

Here 2 is ignored and 100 is matched because it is followed by ₹

let text = `I have 2 notes of 100₹`

let pattern = /\d+\b(?!₹)/gm;
text.match(pattern)

// Output
['2']

Here 100 is ignored and 2 is matched because it is not followed by ₹

Some more to read

Metacharacter	Description
\B	Find a match, but not at the beginning/end of a word
\0	Find a NULL character
\f	Find a form feed character
\xxx	Find the character specified by an octal number xxx
\xdd	Find the character specified by a hexadecimal number dd
\udddd	Find the Unicode character specified by a hexadecimal number dddd. For example \u00A9 → Unicode escaped ©

Quantifiers & Alternation

Metacharacter	Description
^	matches any string at the beginning of it. for example ^i Search for i at the beginning
a* a+ a?	0 or more, 1 or more, 0 or 1
a{5} a{2,}	exactly five, two or more
a{1,3}	between one & three
a+? a{2,}?	match as few as possible
ab|cd	match ab or cd

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