javascript string exec strange behavior

Question

have funciton in my object which is called regularly.

parse : function(html)
{
    var regexp = /...some pattern.../
    var match = regexp.exec(html);
    while (match != null)
    {
        ...
        match = regexp.exec(html);
    }
    ...
    var r = /...pattern.../g;
    var m = r.exec(html);
}

with unchanged html the m returns null each other call. let's say

parse(html);// ok
parse(html);// m is null!!!
parse(html);// ok
parse(html);// m is null!!!
// ...and so on...

is there any index or somrthing that has to be reset on html ... I'm really confused. Why match always returns proper result?