How to use Regular Expression in Javascript

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Author: Doe Hoon LEE

We will first look at some basic regular expression patterns!

#	Pattern	String
1	DAVID	‘DAVID’
2	[DAVID] or D|A|V|I|D	one of D, A, V, I, D
3	[^david]	without D, A, V, I, D
4	^[DAVID]	starting with one of D, A, V, I, D
5	^	indicates string start
6	$	indicates end of string
7	\d or [0-9]	number
8	{n}	n of
9	{n1,n2}	n1 ~ n2

Let’s try using these in real examples then!

const test1 = /DAVID/;

const test2_1 = /[DAVID]/;

const test2_2 = /D|A|V|I|D/;

const test3 = /[^DAVID]/;

const test4 = /^[DAVID]/;

const test_5_and_7_and_9 = /^[A-Z]{2}[0-9]{6}[A-Z]{2}/;

const string1 = "DAVID";

const string2 = "NOT DAVID";

const string3 = "APPLE";

const string4 = "PILL";

const string5 = "KR035720KS";

test1.test(string1); // => true

test2_1.test(string2); // => true

test2_2.test(string2); // => true

test3.test(string3); // => true

test4.test(string4); // => false

test_5_and_7_and_9.test(string5); // => true

Here are some fun techniques you can use with regex!

Remove consecutive/repeating pattern

// let str = "dd_aaa_vvvv_iiiii_dddddd";

return str.replace(/(.)\1+/g, "$1");

// => "d_a_v_i_d"

Cut every nth letters

// cut every single letter
"ABCDEFGHIJKLMNOPQRSTUVWXYZ".match(/.{1,1}/g);
// => ["A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z"]

// cut every TWO letters
"ABCDEFGHIJKLMNOPQRSTUVWXYZ".match(/.{1,2}/g);
// => ["AB", "CD", "EF", "GH", "IJ", "KL", "MN", "OP", "QR", "ST", "UV", "WX", "YZ"]

Find alphabets only!

let str = "1A2BC3DEF4GDSAF84";
str.match(/[A-Z]+/g);
// => ["A", "BC", "DEF", "GDSAF"]

Find numbers only!

let str = "abc123";
let strTwo = "123";

/^\d+$/.test(str) // => false
/^\d+$/.test(strTwo) // => true