正则表达式：以^开始，$结束

/验证密码是6_{20位之间的a}z、A~Z 0-9

            //验证密码是6~20位之间的a~z、A~Z 0-9
            // var reg = new RegExp(/^[\w*&%$#_@]{6,20}$/);
            //身份证15 18 17xX
            // var reg = new RegExp(/^\d{15}$|^\d{18}$|^\d{17}[x|X]{1}$/);
            //英文数字._  @ 英文数字    .   英文数字
            var reg = new RegExp(/^([\w._]+)@(\w+)\.(\w+)$/);
            if(reg.test("12345678901234567x")){
                alert("正确");
            }else{
                alert("错误");
            }

<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title></title>
        <script type="text/javascript">
            function validate(){
                var username = document.getElementById("username").value;
                var password = document.getElementById("password").value;
                var repassword = document.getElementById("repassword").value;
                var userid = document.getElementById("userid").value;
                var email = document.getElementById("email").value;
                // alert(username);
                
                var reg = new RegExp(/^\w{6,20}$/);
                if(!reg.test(username)){
                    alert("用户名错误");
                    return false;
                }
                var pwReg = new RegExp(/^[\w_]{6,20}$/);
                if(!pwReg.test(password)){
                    alert("密码输入错误");
                    return false;
                }
                if(password!=repassword){
                    alert("两次密码不一致");
                    return false;
                }
                
                
                
                
                return true;
            }
        </script>
    </head>
    <body>
        <form action="#"  id="form1" onsubmit="return validate();">
            用户名：<input type="text" name="username" id="username"  /><br>
            密码：<input type="password" name="password" id="password" /><br>
            重复密码：<input type="password" name="password" id="repassword" /><br>
            身份证号码：<input type="text" name="userid" id="userid"  /><br>
            邮箱：<input type="text" name="email" id="email"  /><br>
            <input type="submit"  value="提交" />
            <input type="reset"  value="重置" />
            
        </form>
    </body>
</html>