Question

How do I write a regex to replace a with b and b with a so that a&b will become b&a?

UPDATE

It should replace all occurrences of a with b and each b with a. The a&b is just an example to illustrate what I want. Sorry for the confusion.

Answer 1

The traditional generic way is:

my $string = "abcdefedcba";

my %subst = ( 'a' => 'b', 'b' => 'a' );
$string =~ s/(@{[ join '|', map quotemeta, sort { length($b) <=> length($a) } keys %subst ]})/$subst{$1}/g;

print $string;

Non-generically:

$string =~ s/(a|b)/{a=>'b',b=>'a'}->{$1}/ge;