Flickr replace url with regular expression. Help!

Flickr replace url with regular expression. Help!


I'm trying to use a regular expression to change the url string from
flickr. I want to print the big "_b" images but get an error "replace
is not a function". Any ideas?
$.getJSON("http://api.flickr.com/services/feeds/groups_pool.gne?
id=81799453@N00&lang=en-us&size=b&format=json&jsoncallback=?",
         function(data){
         $.each(data.items, function(i,item) {
                    var regexp = /_m\.jpg/g;
         $("<img/>").attr("src", item.media.m).replace(regexp,
'_b.jpg').appendTo("#images").fadeTo(1800, 1.0);
         if ( i == 8 ) return false;
         });
                    $('#images, .content ul').innerfade({
                        speed: '800',
                        timeout: 5000,
                        type: 'sequence'
                        });
         });