$ perl -le'my $fromUserNumber = shift; $_ = " a b c d "; /(.) (.) (.) (.)/ and print substr $_, $-[$fromUserNumber], $+[$fromUserNumber] - $-[$fromUserNumber]'  2
b

$ perl -le'my $fromUserNumber = shift; $_ = " a b c d "; /(.) (.) (.) (.)/ and print substr $_, $-[$fromUserNumber], $+[$fromUserNumber] - $-[$fromUserNumber]'  3
c