This post explains the basics of Regex. As a javascript developer, it’s very important to know how to perform basic Regex and Solve problems. It’s a universal concept. This post focuses on Regex with javascript.

Below are the different ways to create a Regex expression with javascript.

var re = /ar/;
var re = new RegExp('ar'); 

We can match strings using exec function after defining regex.

re.exec('car'); // returns ["ar", index: 1, input: "car"]
re.exec('cab'); // returns null

There are few special character classes which allow us to write complex regular expressions.

There are many types of elements in RegEx. Some of them are:

• Characters Ex: \w — Alphanumeric, \d — Decimal, \D — Non decimal
• Character classes Ex: [x-y] in range x to y, [^x] not x
• Quantifiers Ex: +, ?, * (greedy and lazy matchers)
• Boundaries Ex: ^(beginning of input), $(end of input)

/* Character class */
var re1 = /[AEIOU]/;
re1.exec("Oval"); // returns ["O", index: 0, input: "Oval"]
re1.exec("2456"); // null
var re2 = /[1-9]/;
re2.exec('mp4'); // returns ["4", index: 2, input: "mp4"]

/* Characters */
var re4 = /\d\D\w/;
re4.exec('1232W2sdf'); // returns ["2W2", index: 3, input: "1232W2sdf"]
re4.exec('W3q'); // returns null

/* Boundaries */
var re5 = /^\d\D\w/;
re5.exec('2W34'); // returns ["2W3", index: 0, input: "2W34"]
re5.exec('W34567'); // returns null
var re6 = /^[0-9]{5}-[0-9]{5}-[0-9]{5}$/;
re6.exec('23451-45242-99078'); // returns ["23451-45242-99078", index: 0, input: "23451-45242-99078"]
re6.exec('23451-abcd-efgh-ijkl'); // returns null

/* Quantifiers */
var re7 = /\d+\D+$/;
re7.exec('2abcd'); // returns ["2abcd", index: 0, input: "2abcd"]
re7.exec('23'); // returns null
re7.exec('2abcd3'); // returns null
var re8 = /<([\w]+).*>(.*?)<\/\1>/;
re8.exec('<p>Hello JS developer</p>'); //returns  ["<p>Hello JS developer</p>", "p", "Hello JS developer", index: 0, input: "<p>Hello JS developer</p>"]

Along with exec, there are other functions namely match, search and, replace are available for finding a string in another using regular expressions. But these functions should be used on the string itself.

"2345-678r9".match(/[a-z A-Z]/); // returns ["r", index: 8, input: "2345-678r9"]
"2345-678r9".replace(/[a-z A-Z]/, ""); // returns 2345-6789

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