Regular expression syntax of (a)

1. JS string is determined whether all the letters

val = "123456"
var isletter = /^[a-zA-Z]+$/.test(val);
document.write(isletter);//false
document.write("<br>");

val2 = "asaaa"
var isletter2 = /^[a-zA-Z]+$/.test(val2);
document.write(isletter2);//true

2.JS string is determined whether all numbers

val = "123456"
var isnum = /^\d+$/.test(val);
document.write(isnum);//true
document.write("<br>");

val2 = "as123"
var isnum2 = /^\d+$/.test(val2);
document.write(isnum2);//false

3.JS determines whether the input numbers, letters, underscores

function isValid(str) {

　　return /^\w+$/.test(str);

}
str = "1234abd__"
document.write(isValid(str));//true
document.write("<br>");

str2 = "$32343#"
document.write(isValid(str2));//false
document.write("<br>");