Although personally I'd still use a conditional, of course it's possible to do it all in one regex. One way is by making the comma optional by putting a ? on a group, in this case I'm using a non-capturing (?:...) group, and I had to make the first part of the regex non-greedy so that it doesn't swallow an existing comma:
use warnings;
use strict;
use Test::More;
my $regex = qr/ ^ (.*?) (?: , ([^,]*) )? $ /x;
ok "abc"=~$regex;
is $1, "abc";
is $2, undef;
ok "abc,5"=~$regex;
is $1, "abc";
is $2, 5;
ok "a,b,c,5"=~$regex;
is $1, "a,b,c";
is $2, 5;
done_testing;
Update: An alternative that says a little more explicitly: either match a string with no commas in it, or, if there are commas, I want to match the thing after the last one: /^ (?| ([^,]*) | (.*) , ([^,]*) ) $/x Update 2: And it turns out this regex is much faster than the above! (try using it in this benchmark)