var str = ("div#demo.demo");
str.replace(/^(\w+)#(\w+)\.(\w+)$/,function(match,$1,$2,$3){
return "<"+$1+" id=\"" + $2+ "\" class=\""+ $3+ "\"><\/" + $1 +">"
})
console.log(str);
var str = ("div#demo.demo").replace(/^\b(\w+)\b#\b(\w+)\b\.\b(\w+)\b$/,function(match,$1,$2,$3){
return "<"+$1+" id=\"" + $2+ "\" class=\""+ $3+ "\"><\/" + $1 +">"
})
console.log(str);