It looks like you need something like this:

$ 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
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