what is the use of regular expression in php code example

Example 1: regex php

<?php
// First Verif your regex code with https://regex101.com/
$str = "Visit W3Schools";
$pattern = "/w3schools/i";
echo preg_match($pattern, $str); // Outputs 1

// test email with REGEX
if (!preg_match("/[-0-9a-zA-Z.+_]+@[-0-9a-zA-Z.+_]+.[a-zA-Z]{2,4}/", $emailAddress)){
    //Email address is invalid.
}

// use filter var to valide Email
if(filter_var($emailAddress, FILTER_VALIDATE_EMAIL)) 
{
     //The email address is valid.
} else{
     //The email address is invalid.
}


?>

Example 2: php regular expression

Modifier	Description
i 	Makes the match case insensitive
m 	Specifies that if the string has newline or carriage
	return characters, the ^ and $ operators will now
	match against a newline boundary, instead of a
	string boundary
o 	Evaluates the expression only once
s 	Allows use of . to match a newline character
x 	Allows you to use white space in the expression for clarity
g 	Globally finds all matches
cg 	Allows a search to continue even after a global match fails