use Parse::RecDescent;
my $teststr="a,b,op2(c,d),op3(e,op4(f,g))";
my $grammar = q {
        content:        /[^\)\(\,]+/
        function:       content '(' list ')'
        value:          content
        item:           function | value
        list:           item ',' list | item
        startrule:      list
};
my $parser = new Parse::RecDescent ($grammar) or die "Bad grammar!\n";
defined $parser->startrule($teststr) or print "Bad text!\n";