Regex expression validation is not working

Hi, 

The regex validation in the below code is not working. could someone please help me on this .

var nonPrint = "123Test*input";
gs.print('nonPrint1'+nonPrint);
var regex =/^[0-9][0-9]*$/;
var s = nonPrint.replace(regex, '_');
gs.print('nonPrint2' + s);

Appreciate for the leads.

Thanks