javascript/JS常用正则表达式

javascript/JS限制用户名只能输入字母和数字下划线

function checkreg(){

        var msg
= "对不起，输入错误\n";

        var
regform = document.all.reg;

        var
username = regform.username;

        var
password1 = regform.password1;

        var
password2 = regform.password2;

        var
usern = /^[a-zA-Z0-9_]{1,}$/;

        //alert(username.value);

        if
(!username.value.match(usern)) {

                msg
+= "用户名只能由字母数字下划线组成\n";

                alert(msg);

                username.value
= '';

                username.focus();

                return
false;

        }

        if(password1.value
!= password2.value)

        {

                msg
+= '两次输入密码不一致';

                alert(msg);

                password1.value
= '';

                password2.value
= '';

                password1.focus();

                return
false;

        }