[ksa]

data:image/jpeg;base64,/9j/4AAQSkZJRgABAQEASABIAAD/2wBDA.........

И мои 5коп...

var src='data:image/jpeg;base64,/9j/4AAQSkZJRgABAQEASABIAAD/2wBDA.........';
alert(src.match(/[^\/]+(?=;)/))