REGEXP

FLAGS

Flag Description
g global match
i ignore case
m multiline, makes ^ and $ match lines rather than the whole string
u unicode
y sticky 
let phrase = '\nIs th\nis h\nis?';
let pattern = /is/;
console.log(phrase.match(pattern)); //=> [ 'is', index: 7, input: '\nIs th\nis h\nis?' ]

Global match

let phrase = '\nIs th\nis h\nis?';
let pattern = /is/g;
console.log(phrase.match(pattern)); //=> [ 'is', 'is' ]

Ignore case

let phrase = '\nIs th\nis h\nis?';
let pattern = /is/gi;
console.log(phrase.match(pattern)); //=> [ 'Is', 'is', 'is' ]

Multiline

let phrase = '\nIs th\nis h\nis?';
let pattern = /^is/gi;
console.log(phrase.match(pattern)); //=> null

let phrase = '\nIs th\nis h\nis?';
let pattern = /^is/gim;
console.log(phrase.match(pattern)); //=> [ 'Is', 'is', 'is' ]

Unicode

let phrase = '\nIs th\nis h\nis?';
let pattern = /^\u{69}s/gim; // '\u{69}' == 'i'
console.log(phrase.match(pattern)); //=> null

let phrase = '\nIs th\nis h\nis?';
let pattern = /^\u{69}s/gimu;
console.log(phrase.match(pattern)); //=> [ 'Is', 'is', 'is' ]

Sticky

let phrase = '\nIs th\nis h\nis?';
let pattern = /is/gi;
console.log(phrase.match(pattern)); //=> [ 'Is', 'is', 'is' ]
console.log(pattern.lastIndex); //=> 0

let phrase = '\nIs th\nis h\nis?';
let pattern = /is/iy;

console.log(phrase.match(pattern)); //=> null
console.log(pattern.lastIndex); //=> 0
pattern.lastIndex = 1;

console.log(phrase.match(pattern)); //=> [ 'Is', index: 1, input: '\nIs th\nis h\nis?' ]
console.log(pattern.lastIndex); //=> 3

console.log(phrase.match(pattern)); //=> null
console.log(pattern.lastIndex); //=> 0

let phrase = '\nIs th\nis h\nis?';
let pattern = /is/iy;

pattern.lastIndex = 0;
console.log(pattern.exec(phrase)); //=> null
console.log(pattern.lastIndex); //=> 0

pattern.lastIndex = 1;
console.log(pattern.exec(phrase)); //=> [ 'Is', index: 1, input: '\nIs th\nis h\nis?' ]
console.log(pattern.lastIndex); //=> 3

pattern.lastIndex = 2;
console.log(pattern.exec(phrase)); //=> null
console.log(pattern.lastIndex); //=> 0

pattern.lastIndex = 7;
console.log(pattern.exec(phrase)); //=> [ 'is', index: 7, input: '\nIs th\nis h\nis?' ]
console.log(pattern.lastIndex); //=> 9

References:

CHARACTER CLASSES / METACHARACTERS

Dot .

matches any single character except line terminators: \n, \r, \u2028 or \u2029.

let message = 'let lot lt';
let pattern = /l.t/g;
console.log(message.match(pattern)); //=> [ 'let', 'lot' ]

Digit \d

[0-9]

let message = 'ifpb1 ifpb2 ifpb@';
let pattern = /ifpb\d/g;
console.log(message.match(pattern)); //=>  [ 'ifpb1', 'ifpb2' ]

Not digit \D

[^0-9]

let message = 'ifpb1 ifpb2 ifpb@';
let pattern = /ifpb\D/g;
console.log(message.match(pattern)); //=> [ 'ifpb@' ]

Word \w

[A-Za-z0-9_]

let message = 'w@ a@ ?@';
let pattern = /\w@/g;
console.log(message.match(pattern)); //=> [ 'w@', 'a@' ]

Not word \W

[^a-za-z0-9_]

let message = 'w@ a@ ?@';
let pattern = /\W@/g;
console.log(message.match(pattern)); //=> [ '?@' ]

Whitespace \s

single white space character, including space, tab, form feed, line feed and other Unicode spaces
> [ \f\n\r\t\v\u00a0\u1680\u180e\u2000-\u200a\u2028\u2029\u202f\u205f\u3000\ufeff]

let message = 'a a aba';
let pattern = /a\sa/g;
console.log(message.match(pattern)); //=> [ 'a a' ]

Not whitespace \S

let message = 'a a aba';
let pattern = /a\Sa/g;
console.log(message.match(pattern)); //=> [ 'aba' ]

Escape \

let message = 'a.a aba';
let pattern = /a\.a/g;
console.log(message.match(pattern)); //=> [ 'a.a' ]

CHARACTER SET

character set []

let message = 'abcdefghijklmnopqrstuvwxyz';
let pattern = /[xyz]/g;
console.log(message.match(pattern)); //=> [ 'x', 'y', 'z' ]

let message = 'abcdefghijklmnopqrstuvwxyz';
let pattern = /[a-c]/g;
console.log(message.match(pattern)); //=> [ 'a', 'b', 'c' ]

let message = 'abcdefghijklmnopqrstuvwxyz0123456789';
let pattern = /[a-c12]/g;
console.log(message.match(pattern)); //=> [ 'a', 'b', 'c', '1', '2' ]

let message = 'abcdefghijklmnopqrstuvwxyz0123456789';
let pattern = /[0-9]/g; // \d
console.log(message.match(pattern)); //=> [ '0', '1', '2', '3', '4', '5', '6', '7', '8', '9' ]

let message = 'abcdefghijklmnopqrstuvwxyzABC';
let pattern = /[A-Z]/g;
console.log(message.match(pattern)); //=> [ 'A', 'B', 'C' ]

let message = 'abcABCDEFGHIJKLMNOPQRSTUVWXYZ';
let pattern = /[a-z]/g;
console.log(message.match(pattern)); //=> [ 'a', 'b', 'c' ]

let message = '0123abcABC!@#';
let pattern = /[A-Za-z0-9_]/g; // \w
console.log(message.match(pattern)); //=> [ '0', '1', '2', '3', 'a', 'b', 'c', 'A', 'B', 'C' ]

Complemented character set [^]

let message = 'tuvwxyz';
let pattern = /[^xyz]/g;
console.log(message.match(pattern)); //=> [ 't', 'u', 'v', 'w' ]

let message = '0123456789';
let pattern = /[^7-9]/g;
console.log(message.match(pattern)); //=> [ '0', '1', '2', '3', '4', '5', '6' ]

let message = '!@#$qwert';
let pattern = /[^A-Za-z0-9_]/g; // \W
console.log(message.match(pattern)); //=> [ '!', '@', '#', '$' ]
pattern = /[\W]/g;
console.log(message.match(pattern)); //=> [ '!', '@', '#', '$' ]

ALTERNATION

Alternation |

let message = 'abcdefghijklmnopqrstuvwxyz';
let pattern = /x|y/g;
console.log(message.match(pattern)); //=> [ 'x', 'y' ]

let message = 'red green blue';
let pattern = /green|red/g; // != [green|red]
console.log(message.match(pattern)); //=> [ 'red', 'green' ]

let message = 'dark red green blue';
let pattern = /(dark )?(green|red)/g;
console.log(message.match(pattern)); //=> [ 'dark red', 'green' ]

BOUNDARIES

Beginning ^

let message = 'instituto federal';
let pattern = /^instituto/;
console.log(message.match(pattern)); //=> [ 'instituto', index: 0, input: 'instituto federal' ]

let message = 'o instituto federal da paraíba';
let pattern = /^instituto/;
console.log(message.match(pattern)); //=> null

let message = 'lorem\nipsum\ndolor';
let pattern = /^\w{4}/g;
console.log(message.match(pattern)); //=> [ 'lore' ]

let message = 'lorem\nipsum\ndolor';
let pattern = /^\w{4}/gm;
console.log(message.match(pattern)); //=> [ 'lore', 'ipsu', 'dolo' ]

End $

let message = 'instituto federal';
let pattern = /federal$/;
console.log(message.match(pattern)); //=> [ 'federal', index: 10, input: 'instituto federal' ]

let message = 'o instituto federal da paraíba';
let pattern = /federal$/;
console.log(message.match(pattern)); //=> null

Word boundary \b

let message = 'apple, blackberry, cherry';
let pattern = /\ba\w*/g;
console.log(message.match(pattern)); //=> [ 'apple' ]

let message = 'apple, blackberry, cherry';
let pattern = /\w*y\b/g;
console.log(message.match(pattern)); //=> [ 'blackberry', 'cherry' ]

Non-word boundary \B

let message = 'apple, blackberry, cherry, charr';
let pattern = /\w*rr\B\w*/g;
console.log(message.match(pattern)); //=> [ 'blackberry', 'cherry' ]

GROUPING AND BACK REFERENCES

Capturing group ()

let message = 'baby, babies, boom';
let pattern = /bab(y|ies)/g;
console.log(message.match(pattern)); //=> [ 'baby', 'babies' ]

Backreference \n

let message = 'hah dad bad dab gag gab';
let pattern = /(\w)a\1/g;
console.log(message.match(pattern)); //=> [ 'hah', 'dad', 'gag' ]

let message = '<b>lorem</b> and <i>ipsum</i> and <span>dolor</span>';
let pattern = /<(\w+)>(.*)<\/\1>/g;
console.log(message.match(pattern)); //=> [ '<b>lorem</b>', '<i>ipsum</i>', '<span>dolor</span>' ]

References:

Non-capturing group (?:x)

let message = 'http://stackoverflow.com/';
let pattern = /(?:http|ftp):\/\/([^\/\r\n]+)(\/[^\r\n]*)?/;
console.log(message.match(pattern));
//=>
// [ 'http://stackoverflow.com/', 'stackoverflow.com', '/', index: 0, input: 'http://stackoverflow.com/' ]
// $1 'stackoverflow.com'
// $2 '/'

let message = 'http://stackoverflow.com/questions/tagged/regex';
let pattern = /(?:http|ftp):\/\/([^\/\r\n]+)(\/[^\r\n]*)?/;
console.log(message.match(pattern));
//=>
// [
//    'http://stackoverflow.com/questions/tagged/regex',
//    'stackoverflow.com',
//    '/questions/tagged/regex',
//    index: 0,
//    input: 'http://stackoverflow.com/questions/tagged/regex'
//  ]
// $1 'stackoverflow.com'
// $2 '/questions/tagged/regex'

let message = '1st';
let pattern = /([0-9]+)(?:st|nd|rd|th)?/;
console.log(message.match(pattern));
//=>
// [ '1st', '1', index: 0, input: '1st' ]
// $1 '1'

QUANTIFIER

x*

let pattern = /<.*>/g;
let message = '<foo> <bar> <>';
console.log(message.match(pattern)); //=> [ '<foo> <bar> <>' ]

x*?

let pattern = /<.*?>/g;
let message = '<foo> <bar> <>';
console.log(message.match(pattern)); //=> [ '<foo>', '<bar>', '<>' ]

x+

let pattern = /\w+@\w+/g;
let message;

message = 'root@ifpb';
console.log(message.match(pattern)); //=> [ 'root@ifpb' ]
message = 'root@i';
console.log(message.match(pattern)); //=> [ 'root@i' ]
message = 'root@';
console.log(message.match(pattern)); //=> null

let pattern = /\w+/g;
let message = 'root@ifpb';
console.log(message.match(pattern)); //=> [ 'root', 'ifpb' ]

let pattern = /\d+/g;
let message = 'abc12345678cde90fgh';
console.log(message.match(pattern)); //=> [ '12345678', '90' ]

x+?

let pattern = /\d+?/g;
let message = 'abc12345678cde90fgh';
console.log(message.match(pattern)); //=> [ '1', '2', '3', '4', '5', '6', '7', '8', '9', '0' ]

x?

let pattern = /\d?/;
let message = '12';
console.log(message.match(pattern)); //=> [ '1', index: 0, input: '12' ]

x??

let pattern = /\d??/;
let message = '12';
console.log(message.match(pattern)); //=> [ '', index: 0, input: '12' ]

x{n}

let pattern = /\d{5}/g;
let message = 'abc12345678cde90fgh';
console.log(message.match(pattern)); //=> [ '12345' ]

x{n}?

let pattern = /\d{5}?/g;
let message = 'abc12345678cde90fgh';
console.log(message.match(pattern)); //=> [ '12345' ]

x{n,}

let pattern = /\d{5,}/g;
let message = 'abc12345678cde90fgh';
console.log(message.match(pattern)); //=> [ '12345678' ]

x{n,}?

let pattern = /\d{5,}?/g;
let message = 'abc12345678cde90fgh';
console.log(message.match(pattern)); //=> [ '12345' ]

x{n,m}

let pattern = /\d{1,5}/g;
let message = 'abc12345678cde90fgh';
console.log(message.match(pattern)); //=> [ '12345', '678', '90' ]

x{n,m}?

let pattern = /\d{1,5}?/g;
let message = 'abc12345678cde90fgh';
console.log(message.match(pattern)); //=> [ '1', '2', '3', '4', '5', '6', '7', '8', '9', '0' ]

ASSERTION

Positive Lookahead x(?=y)

x only if x is followed by y

let pattern = /\d(?=px)/g;
let message = '1pt 2px 3em 4px 4xp 3';
console.log(message.match(pattern)); //=> [ '2', '4' ]

let pattern = /(?=p)\w+/g;
let message = '1pt 2px 3em 4px 4xp 3';
console.log(message.match(pattern)); //=> [ 'pt', 'px', 'px' , 'p' ]

Negative Lookahead x(?!y)

x only if x is not followed by y

let pattern = /\d(?!px)/g;
let message = '1pt 2px 3em 4px 4xp 3';
console.log(message.match(pattern)); //=> [ '1', '3', '4', '3' ]

let pattern = /(?!p)\d+/g;
let message = '1pt 2px 3em 4px 4xp 3 px';
console.log(message.match(pattern)); //=> [ '1', '2', '3', '4', '4', '3' ]

RECAP

Names Regexp
FLAGS g, i, m, u, y
CHARACTER CLASSES . , \d, \D, \w, \W, \s, \S, \
CHARACTER SET [], [^]
BOUNDARIES ^, $, \b, \B
ALTERNATION |
GROUPING AND BACK REFERENCES (), \n, (?:x)
QUANTIFIERS Greedy (*, +, ?, {n}, {n,}, {n, m}), Non-Greedy (x*?, x+?, x??, x{n}?, x{n,}?, x{n,m}?)
ASSERTIONS x(?=y), x(?!y)

REFERENCES