#!/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)        # get to specific row and column
    (?s:(?<=(.)$b)?)  # previous row ($1)
    (?<=(.))?         # previous column ($2)
    (.)               # this row, this column ($3)
    (?=(.))?          # next column ($4)
    (?s:(?=$f(.))?)   # next row ($5)
  }x;
}