just take a look at the quote in my second post.
and if you don't believe that, you could always benchmark it yourself
Benchmarking proves that statment to be completely untrue by a factor of 10x (even more with long strings).
One other difference is they match 2 different things. PCRE matches the first found left most match whereas POSIX matches the longest left most match.
$regex = 'cat|category';
$target = 'category';
echo '<p>ereg: ' . (ereg($regex, $target, $match)? $match : 'false').'</p>'; // category
echo '<p>preg: ' . (preg_match('/'.$regex.'/', $target, $match)? $match : 'false').'</p>'; // cat