JSFiddle - React, Tailwind, and code Playground

HTML

var patt = /w3schools/i <br>
i for case in case-insensitive search

JavaScript

var alert = function (str) {
    var st = document.createTextNode(str);
    var p = document.createElement('p');
    p.appendChild(st);
    document.querySelector('body').appendChild(p);
}

//regular expression in javascript

/*
A regular expression is a sequence of characters that forms a search pattern.
When you search for data in a text, you can use this search pattern to describe what you are searching for.
*/

var patt = /w3schools/i
/*
/w3schools/i  is a regular expression.
w3schools  is a pattern (to be used in a search).
i  is a modifier (modifies the search to be case-insensitive).
*/


var str = "Visit W3Schools";
var n = str.search(/w3schools/i);   // return postion if found
alert(n)
// we can also do
var n = str.search("W3Schools");
alert(n)


var	str = "Visit Microsoft!";
// this replace function , replace the String and return result string
var res = str.replace(/microsoft/i, "W3Schools");
alert(res)

var str = "Visit W3Schools";
var pattern = /w3schools/i;
var resu = pattern.test(str);
alert(1)
alert(resu)

alert(2)
alert(/abc/.test('abcde'))
alert(/abc/.test('abdec'))

alert(3)
// validation for only 5 digits
alert(/^\d{5}$/.test('12345'))
alert(/^\d{5}$/.test('123456'))
alert(/^\d{5}$/.test('1234F'))
/* ^ indicates the beginning of the string. Using a ^ metacharacter     requires that the match start at the beginning.
   \d indicates a digit character and the {5} following it means that there must be 5 consecutive digit characters.
    $ indicates the end of the string. Using a $ metacharacter requires that the match end at the end of the string.
*/

alert(4)
// digits only
alert(/\d/.test('11111'))
alert(/\d/.test('FFFF'))

alert(5)
// digit not allowed
alert(/[^0-9]/.test('111'))          // false
alert(/[^0-9]/.test('asdasd'))       // true

alert(6)
// characters only
alert(/^[a-zA-Z]+$/.test('abcdes'))
alert(/^[a-zA-Z]+$/.test('abcdes123'))

alert(7)
/* Matches end of input.
For example, /t$/ does not match the 't' in "eater", but does match...