Email address validation in PHP using REGEX

Code to validate:

if (eregi('^[a-zA-Z0-9._-]+@[a-zA-Z0-9._-]+\.([a-zA-Z]{2,4})$',$_REQUEST['email'])) { 
    echo 'Valid'; 
       } else { 
    echo 'Invalid'; 
       }

Regular Expression Samples

Regx Expression for removing repeated words (case insensitive)

$text = preg_replace("/\s(\w+\s)\1/i", "$1", $text);

Output : ‘Keep your your head’ becomes ‘Keep your head’

Password complexity

Tests if the input consists of 6 or more letters, digits, underscores and hyphens.
The input must contain at least one upper case letter, one lower case letter and one digit.

'\A(?=[-_a-zA-Z0-9]*?[A-Z])(?=[-_a-zA-Z0-9]*?[a-z])(?=[-_a-zA-Z0-9]*?[0-9])[-_a-zA-Z0-9]{6,}\z'