/(http|https):\/\/[\w\-_]+(\.[\w\-_]+)+([\w\-\.,@?^
=%&:/~\+#]*[\w\-\@?^=%&/~\+#])?/

And to implement it you can loop through your containers that might contain links

document.observe("dom:loaded", function(){
var posts = $$("div#posts");
for(var i = 0; i < posts.length; i++){
var link_regex = /(http|https):\/\/[\w\-_]+(\.[\w\-_]+)+([\w\-\.,@?^
=%&amp;:/~\+#]*[\w\-\@?^=%&amp;/~\+#])?/;
var parsed_string = posts[i].innerHTML.gsub(link_regex, '<a href="#{0}"
target="_blank">#{0}</a>');
posts[i].innerHTML = parsed_string;
}
});