my $alpha = join '', ('a'..'z');  # $alpha = 'abcdefghijklmnopqrstuvwxyz'
$alpha =~ s/(?<=def).*(?=uvw)//;  # remove everything preceeded by the substring 'def'
                                  #   and succeeded by the substring 'uvw'
print $alpha;                     # $alpha = 'abcdefuvwxyz'