Re: REgular expression to check the string that allows "a","b" and "c" to occur only once in any order.

[snoopy]

Here's a solution based on a hash/regex combo:

my $chars = "abc";

foreach (qw/abc bca cab cba bac acb aab abbc acc/) {

  my %seen;
  my $pat = $_;

  $pat =~ s/([\Q$chars\E])/++$seen{$1}/gex;

  if ((grep {$_ == 1} values %seen) == length($chars)) {
    print "$_ matches\n";
  }
  else{
    print "$_ doesn't match\n";
  }
}