Although you want to implement the solution in c#. Better suggestion would be is to implement in javascript.
Add simple javascript function to in aspx
page and no code in codebehind file.
$(document).ready(function () {
var arrForNum = ['gms', 'rs', 'knot']; //Your list of label texts for Number only textboxes
// Now traverse for all textboxes where u want to add some restrictons
$('body').find('.customonly').each(function () {
var id = this.id;
var res = $('label[for=' + id + ']').text();
// check if its the array we declared else it will be charecters only.
if ($.inArray(res, arrForNum) >= 0) {
$(this).forceNumeric(); // Added simple function in fiddle.
//You can apply any other function here if required.
} else {
$(this).forceCharecters('chars');
}
});
});
Check the JsFiddle for detailed code.