I wrote a program, named 'proximity', that does matrix-type things (2-D stuff, basically) with regexes.

#!/usr/bin/perl
  
$string = << "END";
ABC
DEF  
GHI
END 
    
for $a (0 .. 2){
  for $b (0 .. 2) {
    ($u,$l,$P,$r,$d) = prox($string, $a, $b);
    print "$P: u=$u, l=$l, r=$r, d=$d\n";
  }
}   
     
    
sub prox {
  my ($str,$r,$c) = @_;
  my $rlen = index($str, "\n");
  my $skip = '.' x (($rlen + 1) * $r + $c);
  my $b = '.' x $rlen;
  my $f = '.' x $rlen;
   
  return $str =~ m{
    ^
    (?s:$skip)  
    (?s:(?<=(.)$b)?)
    (?<=(.))?
    (.)
    (?=(.))?
    (?s:(?=$f(.))?)
  }x;
}
[download]

As for variable-length look-behinds... you can implement those via revexes!

$_="goto+F.print+chop;\n=yhpaj";F1:eval