Have you a test case that fails? Why do you think "right to left" will fix the failing test case?

Note that you can reverse a string with reverse then match against the reversed string. But that's only going to help if you can identify the problem you are trying to fix, and reversing the string somehow fixes that problem.

Is there a way to tell regex to work from right to left?

No. One of the basic principles of the regex engine is that it works from left to right (GrandFather's suggestion of reverseing the string is a workaround/hack, though I personally have never seen anyone actually do this in production). Another basic principle is that the engine will stop at the first successful match, which sometimes leads to confusion when, for example, people expect .* to match more than "" (though in your example in the root node you're using the ^ $ anchors to help with that). Combine this with the idea of backtracking (Update: which of course does work from right to left, but too much backtracking can be very inefficient) and hopefully this will lead to a better understanding :-) I very much recommend a read of perlretut, and if you want to see your regex in action, then install Regexp::Debugger and run e.g. perl -MRegexp::Debugger -e '"foo.bar" =~ /^.*(\..*)$/'

I had a hard time thinking of an example where reversing the string before using a regex made sense. I came across this little gem..

use strict;
use warnings;

sub commify {
    my $text = reverse $_[0];
    $text =~ s/(\d\d\d)(?=\d)(?!\d*\.)/$1,/g;
    return scalar reverse $text;
}

foreach (qw(-1000 23000000))
{
   print "$_ transformed to: ",commify($_),"\n";
}

__END__
-1000 transformed to: -1,000
23000000 transformed to: 23,000,000
[download]

There are of course ways to do this with printf().

use strict;
use warnings;

foreach my $test ('..', 'file.txt', 'blah.abc.txt')
{
   my ($suffix) = $test =~ /(\.[^.]+)$/;
   $suffix //= '';  #suffix is null string if no match
   print "test=$test  suffix=$suffix\n";
}

__END__
test=..  suffix=
test=file.txt  suffix=.txt
test=blah.abc.txt  suffix=.txt
[download]

Update: I guess probably: my ($suffix) = $test =~ /(\.\w+)$/;
Word characters are A-Za-z0-9_. Space and control chars are not allowed.