what is regular expression in javascript

1//Declare Reg using slash
2let reg = /abc/
3//Declare using class, useful for buil a RegExp from a variable
4reg = new RegExp('abc')
5
6//Option you must know: i -> Not case sensitive, g -> match all the string
7let str = 'Abc abc abc'
8str.match(/abc/) //Array(1) ["abc"] match only the first and return
9str.match(/abc/g) //Array(2) ["abc","abc"] match all
10str.match(/abc/i) //Array(1) ["Abc"] not case sensitive
11str.match(/abc/ig) //Array(3) ["Abc","abc","abc"]
12//the equivalent with new RegExp is
13str.match('abc', 'ig') //Array(3) ["Abc","abc","abc"]

1// Javascript Regex Reference
2//  /abc/	A sequence of characters
3//  /[abc]/	Any character from a set of characters
4//  /[^abc]/	Any character not in a set of characters
5//  /[0-9]/	Any character in a range of characters
6//  /x+/	One or more occurrences of the pattern x
7//  /x+?/	One or more occurrences, nongreedy
8//  /x*/	Zero or more occurrences
9//  /x?/	Zero or one occurrence
10//  /x{2,4}/	Two to four occurrences
11//  /(abc)/	A group
12//  /a|b|c/	Any one of several patterns
13//  /\d/	Any digit character
14// /\w/	An alphanumeric character (“word character”)
15//  /\s/	Any whitespace character
16//  /./	Any character except newlines
17//  /\b/	A word boundary
18//  /^/	Start of input
19//  /$/	End of input

1// \d	Any digit character
2// \w	An alphanumeric character (“word character”)
3// \s	Any whitespace character (space, tab, newline, and similar)
4// \D	A character that is not a digit
5// \W	A nonalphanumeric character
6// \S	A nonwhitespace character
7// .	Any character except for newline
8// /abc/	A sequence of characters
9// /[abc]/	Any character from a set of characters
10// /[^abc]/	Any character not in a set of characters
11// /[0-9]/	Any character in a range of characters
12// /x+/	One or more occurrences of the pattern x
13// /x+?/	One or more occurrences, nongreedy
14// /x*/	Zero or more occurrences
15// /x?/	Zero or one occurrence
16// /x{2,4}/	Two to four occurrences
17// /(abc)/	A group
18// /a|b|c/	Any one of several patterns
19// /\d/	Any digit character
20// /\w/	An alphanumeric character (“word character”)
21// /\s/	Any whitespace character
22// /./	Any character except newlines
23// /\b/	A word boundary
24// /^/	Start of input
25// /$/	End of input

1//Adding '/' around regex
2var regex = /\s/g;
3//or using RegExp
4var regex = new RegExp("\s", "g");

1let re = /ol/, 
2Characters \, ., \cX, \d, \D, \f, \n, \r, \s, \S, \t, \v, \w, \W, \0, \xhh, \uhhhh, \uhhhhh, [\b]	
3Assertions 	^, $, x(?=y), x(?!y), (?<=y)x, (?<!y)x, \b, \B
4Groups 		(x), (?:x), (?<Name>x), x|y, [xyz], [^xyz], \Number	
5Quantifiers *, +, ?, x{n}, x{n,}, x{n,m}
6Unicode \p{UnicodeProperty}, \P{UnicodeProperty}	 property escapes
7let defaults = new RegExp('compiled'); 
8defaults = { dotAll: false, flags: "", global: false, ignoreCase: false, falselastIndex: 0, multiline: false, source: "abc", sticky: false, unicode: false}

1// Regular expression match sequence of characters
2console.log(/abc/.test("abcde")); // true
3console.log(/abc/.test("abxde")); // false