javascript判断输入的url是否正确

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<html> 
<head> 
<title>检查是否为URL</title> 
<script language="javascript"> 
function check(){ 
str = form1.txt.value; 
str = str.match(/http:\/\/.+/); 
if (str == null){ 
alert('你输入的URL无效'); 
return false; 
}else{ 
alert("你输入的URL有效"); 
return true; 
} 
} 
</script> 


</head> 
<body> 
<form name="form1" onSubmit="return check()"> 
<input type="text" name="txt"> 
<input type="submit" value="检验"> 
</form> 
</body> 
</html>