for ("42,43", "41,42", "41,42,43", "41,142,423") {
  print "$_ \t=> ";
  my $string = $_;
  $string =~ s/(
                ((?=,)|^)42(,|$)
                 |
                (,|^)42((?=,)|$)
               )
              //xg;
  print "$string\n";
}

__OUTPUT__
42,43        => 43
41,42        => 41
41,42,43     => 41,43
41,142,423   => 41,142,423