php regex

<?php
//Syntex : int preg_match( $pattern, $input, $matches, $flags, $offset)
  
// Declare a variable and initialize it 
$str = "Check For Testing."; 
  
// case-Insensitive search for the word "Check" 
if (preg_match("/\bCheck\b/i", $str, $match))  
    echo "Matched!"; 
else
    echo "not matched";   
  

// Output : Matched
?>

preg_match('/(foo)(bar)(baz)/', 'foobarbaz', $matches, PREG_OFFSET_CAPTURE);

if(!preg_match('/^\[a-zA-Z]+$/',$input)) {
   // String contains not allowed characters ...
}

<?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.
}


?>

if (preg_match("/\bweb\b/i", "PHP is the web scripting language of choice.")) {
    echo "A match was found.";
} else {
    echo "A match was not found.";
}

preg_match ( string $pattern , string $subject [, array &$matches [, int $flags = 0 [, int $offset = 0 ]]] ) : int

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