$s="a1 a2 a3";

for (1..2) { 
    print $1 if $s =~ ?a(\d+)?g;
}

##</code><code>##

$s="a1 a2 a3";

print $1 if $s =~ ?a(\d+)?g;
print $1 if $s =~ ?a(\d+)?g;