You didn't tell us clearly what you are trying to discover about the string. It sounds like you want to test that it starts with a letter, and that everything after that is a letter, underscore or digit. But it's not very clear.
Regex is tricky and expensive, and only easy to read and understand for simple cases. And for simple cases you don't need it anyway. For example, if your requirement is "The string begins with an underscore, every character after that is one of 0-9, and the string is 10 chars in length", you can do that as follows. Just comment what your code is checking for.
console.log( f("_041827492") );
console.log( f("_04139142i") );
function f(str){
// Are all the following true?
// str is ten chars in length
// str begins with underscore
// every subsequent char is a digit
var result = str.length == 10
&& str.split('')[0] == '_'
&& str.substring(1)
.split('')
.filter( function(e) { return ['0','1','2','3','4','5','6','7','8','9'].indexOf(e) == -1; } ) // remove everything not a digit
.join('')
.length == 0;
return result;
}
C:\temp>node ex.js
true
false
