<?php

$username = "user gmail.com";

if (!preg_match('/^[\w-.@]+$/', $username)) {
   echo 'nao passou';
} else{
   echo 'passou';
}

echo ' / ';

$username = "user@_gmail.com";

if (!preg_match('/^[\w-.@]+$/', $username)) {
   echo 'nao passou';
}else{
   echo 'passou';
}