<?php
$a = 'hello @user how are you';
$b = 'RT @user how are you';
$regex = '/(?<!RT)\s@([a-z0-9]{1,20})/i';

$aa = preg_match('/(?<!RT)\s@([a-z0-9]{1,20})/i', $a);

var_dump($aa);

$bb = preg_match('/(?<!RT)\s@([a-z0-9]{1,20})/i', $b);

var_dump($bb);