<?php
$pattern = "/^([_\.0-9a-z]+-*[_\.0-9a-z]*)+@([0-9a-z][_0-9a-z]+-*[_0-9a-z]*\.)+[a-z]{2,3}$/i";

$emails = array("email-@example.com",
"email-@example.com",
"email.2@example.com",
"email@example.com.vg",
"email_@example.us",
"@example.us",
"email@example.",
"email@example",
"test.email@example.us",
"a@",
"my_email@example.com");

foreach ($emails AS $email)
{
    echo $email;
    echo (preg_match($pattern, $email)) ? " valid" : " unvalid";
    echo "<br>";
}


/*
// output:
email-@example.com valid
email-@example.com valid
email.2@example.com valid
email@example.com.vg valid
email_@example.us valid
@example.us unvalid
email@example. unvalid
email@example unvalid
test.email@example.us valid
a@ unvalid
my_email@example.com valid
*/