<form name="myform" action="loginchk.php" method="post" onSubmit="return CheckForm();">

    <input type="submit" name="Submit" value="登陆">

</form> 

function CheckForm(){    

    var sex= document.myform.sex.value;

    var title= document.myform.title.value;

    var tel= document.myform.tel.value;

     if(sex==""){

     alert("性别不能为空！");

     return false;

     }

    if(title.length<5){

     alert("标题不能小于5个字！");

     return false;

    }

    var myreg = /^[1][3,4,5,7,8,9][0-9]{9}$/;

   if (!myreg.test(tel)) {

     alert("手机号不正确！");

     return false;

     }

}