sub compatible { my ($s1, $s2) = @_; my $m = length($s1) - 1; my($n, $p) = ($m - 1, $m + 1); ($s1 . $s2) !~ /^.{0,$m}?([^_]).{$m}(?!\1)[^_]/ and ($s1 . $s2) !~ /^.{0,$m}?([^_])(?:.{0,$n}|.{$p,})\1/; } printf "%s v %s ? %s\n", @$_, compatible( @$_ ) ? 1 : 0 for [ qw[ _8__3__19 48____7__ ] ], # compat [ qw[ _8__3__19 4_8___7__ ] ], # clash [ qw[ __8_3__19 48____7__ ] ], # clash [ qw[ __8_3__19 84____7__ ] ], # clash [ qw[ _8__3__19 48_____7_ ] ]; # clash