Javascript Regular Expressions

From the MDN web docs

Regular expressions are patterns used to match character combinations in strings. In JavaScript, regular expressions are also objects. These patterns are used with the exec() and test() methods of RegExp, and with the match(), matchAll(), replace(), replaceAll(), search(), and split() methods of String. This chapter describes JavaScript regular expressions.

Regular Expression Symbols

Symbol	Description
`.`	Matches any character except newline
`\d`	Matches any digit (0-9)
`\D`	Matches any non-digit character
`\s`	Matches any whitespace character
`\S`	Matches any non-whitespace character
`\w`	Matches any alphanumeric character (a-z, A-Z, 0-9) and underscore (_)
`\W`	Matches any non-alphanumeric character
`[abc]`	Matches any character inside the brackets (a, b, or c)
`[^abc]`	Matches any character not inside the brackets
`^`	Matches the start of the string
`$`	Matches the end of the string
`*`	Matches zero or more occurrences of the preceding element
`+`	Matches one or more occurrences of the preceding element
`?`	Matches zero or one occurrence of the preceding element
`{n}`	Matches exactly n occurrences of the preceding element
`{n,}`	Matches n or more occurrences of the preceding element
`{n,m}`	Matches at least n and at most m occurrences of the preceding element
`\|`	Acts as a boolean OR. Matches the pattern before or the pattern after the \|
`( )`	Defines a group
`(?: )`	Defines a non-capturing group
`(?= )`	Positive lookahead
`(?! )`	Negative lookahead

Complex Patterns

Please note that following examples are simple examples and regular expressions can be combined to create more complex patterns.

Dot (.)

Matches any character except newline.

let regex = /a.c/;
let str = 'abc';
console.log(regex.test(str)); // true

Digit (\d)

Matches any digit (0-9).

let regex = /\d/;
let str = 'abc123';
console.log(regex.test(str)); // true

Non-Digit (\D)

Matches any non-digit character.

let regex = /\D/;
let str = 'abc123';
console.log(regex.test(str)); // true

Whitespace (\s)

Matches any whitespace character.

let regex = /\s/;
let str = 'abc def';
console.log(regex.test(str)); // true

Non-Whitespace (\S)

Matches any non-whitespace character.

let regex = /\S/;
let str = ' abc';
console.log(regex.test(str)); // true

Word Character (\w)

Matches any alphanumeric character (a-z, A-Z, 0-9) and underscore (_).

let regex = /\w/;
let str = 'abc';
console.log(regex.test(str)); // true

Non-Word Character (\W)

Matches any non-alphanumeric character.

let regex = /\W/;
let str = 'abc-def';
console.log(regex.test(str)); // true

Character Set ([abc])

Matches any character inside the brackets (a, b, or c).

let regex = /[abc]/;
let str = 'defabc';
console.log(regex.test(str)); // true

Negated Character Set ([^abc])

Matches any character not inside the brackets.

let regex = /[^abc]/;
let str = 'defabc';
console.log(regex.test(str)); // true

Start Anchor (^)

Matches the start of the string.

let regex = /^abc/;
let str = 'abcdef';
console.log(regex.test(str)); // true

End Anchor ($)

Matches the end of the string.

let regex = /def$/;
let str = 'abcdef';
console.log(regex.test(str)); // true

Zero or More (*):** Matches zero or more occurrences of the preceding element.

let regex = /a*/;
let str = 'aaaabc';
console.log(regex.test(str)); // true

One or More (+)

Matches one or more occurrences of the preceding element.

let regex = /a+/;
let str = 'aaaabc';
console.log(regex.test(str)); // true

Zero or One (?)

Matches zero or one occurrence of the preceding element.

let regex = /a?/;
let str = 'abc';
console.log(regex.test(str)); // true

Exactly N ({n})

Matches exactly n occurrences of the preceding element.

let regex = /a{2}/;
let str = 'aaaabc';
console.log(regex.test(str)); // true

N or More ({n,})

Matches n or more occurrences of the preceding element.

let regex = /a{2,}/;
let str = 'aaaabc';
console.log(regex.test(str)); // true

Between N and M ({nm})

Matches at least n and at most m occurrences of the preceding element.

let regex = /a{2,3}/;
let str = 'aaaabc';
console.log(regex.test(str)); // true

OR (|)

Acts as a boolean OR. Matches the pattern before or the pattern after the |.

let regex = /abc|def/;
let str1 = 'abc';
let str2 = 'def';
console.log(regex.test(str1)); // true
console.log(regex.test(str2)); // true

Grouping (()

Defines a group.

let regex = /(abc)/;
let str = 'abcdef';
console.log(regex.test(str)); // true

Non-Capturing Group ((?: ))

Defines a non-capturing group.

let regex = /(?:abc)/;
let str = 'abcdef';
console.log(regex.test(str)); // true

Sure, here is the continuation from item 21:

Positive Lookahead ((?= ))

Positive lookahead.

let regex = /abc(?=def)/;
let str = 'abcdef';
console.log(regex.test(str)); // true

Negative Lookahead ((?! ))

Negative lookahead.

let regex = /abc(?!def)/;
let str = 'abcghi';
console.log(regex.test(str)); // true