"1" =~ /1/    or die;
print $#-?"yes\n":"no\n";  # -> no
"1" =~ /(1)?/ or die;
print $#-?"yes\n":"no\n";  # -> yes
"1" =~ /(2)?/ or die;
print $#-?"yes\n":"no\n";  # -> no !!
"1" =~ /(2)?/ or die;
print $#+?"yes\n":"no\n";  # -> yes