Performance question if vs ereg_replace

[zevious]

I am curious as to which of the following would be faster? This is part of a large loop and a slight difference in this will make a big difference in the overall speed.

if($kk==1) $stt[1]=ereg_replace("[^{0-9]","",$stt[1]);}

or just let it perform this everytime through the loop:

$stt[1]=ereg_replace("[^{0-9]","",$stt[1]);}