[jQuery] Replace not a function with Regular Expression

[jQuery] Replace not a function with Regular Expression


Trying to pull in flickr feed and replace the _m with _b in the url to
print large images.
        $.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) {
         $("<img/>").attr("src", item.media.m).match(/_m\.jpg/g,
'_b.jpg').appendTo("#images").fadeTo(1800, 1.0);
         if ( i == 8 ) return false;
         });
                    $('#images, .content ul').innerfade({
                        speed: '800',
                        timeout: 5000,
                        type: 'sequence'
                        });
         });
I'm having a tough time with the regular expression. I get replace is
not a function. Anyone have any ideas on how to replace the url _m
with _b?