$_="a1 a2 a3";
while(?a(\d)?g){ print $1; }

##</code><code>##

$_="a1 a2 a3";
while(?a(\d)?g){ print $1; }
while(?a(\d)?g){ print $1; }

##</code><code>##

$_="a1 a2 a3 a4";
while(/a(\d)/g){ print $1; last; }
while(/a(\d)/g){ print $1; }