rollovers = function( off, on ){
  var img = document.getElementsByTagName("img"); 
  var p = []; // プリロードようの配列
  var off_reg = new RegExp(off+"(\.[a-z]+$)","i"); 
  var on_reg = new RegExp(on+"(\.[a-z]+$)","i" ); 
  for(var x=0,i;i=img[x];x++){ 
    if(i.src.match(off_reg)){ 
      p[x] = new Image(); 
      p[x].src = i.src.replace(off_reg, on+"$1"); 
      i.onmouseover = function(){ this.src = this.src.replace(off_reg,on+"$1"); }; 
      i.onmouseout = function(){ this.src = this.src.replace(on_reg,off+"$1"); }; 
    };
  };
};

window.onload = function(){
  
  rollovers( "_o" , "_h" );
}