无刷新验证表单

<script type="text/javascript">
  function checklogin1(){
    if(document.login1.username.value=="" || document.login1.username.value.replace(/s/g,'')==''){
      alert('请输入用户名！');
      document.login1.username.focus();
      return false;
    }else if(document.login1.password.value==""){
      alert('请输入密码！');
      document.login1.password.focus();
      return false;
    }else if(document.login1.answer.value==""){
      alert('请输入问题验证！');
      document.login1.answer.focus();
      return false;
    }else if(document.login1.answer.value !="199"){
      alert('问题验证错误！');
      document.login1.answer.focus();
      return false;
    }

  }
</script>

<form name="login1" method="post" action=" " onsubmit="return checklogin1();">
    <div class="regc">            
            <span class="left">用户名：</span><input class="inputstyle1" type="text" id="username" name="username" />
            <span class="left">密码：</span><input class="inputstyle1" type="password" id="password" name="password" />
            <span class="left">问题验证</span><input type="text" name="answer" id="answer" size="20" class="textInt inputstyle1"/>
            <input type="submit" id="submit" name="submit" value="登录"  class="newFormBtn_red2"/>
    </div>
</form>

去除空格

 document.login1.username.value.replace(/s/g,'')==''

 PHP补充：

if(mysql_affected_rows()>0){
    echo "<script>alert('用户名已存在');window.history.go(-1);</script>";
    // header("refresh:0;url=./register.php");
    die;
}

window.history.go(-1);也可避免刷新后表单中数据丢失