Either change the field type to integer or try this on-chance client script:
var val = newValue;
var achar = '0123456789.'; //Values you want to check for in this have only allowed number but you could to special characters or letter
var failmsg = 'Enter Here the message you wish to display to the user.';
//create an array that is all lowercase based on the value if you want this case sensitive remove .toLowerCase()
var array1 = val.toLowerCase().split('');
for (var i=0; i < array1.length; i++){
if (achar.indexOf(array1[i]) >= '0'){
}
else{
alert(failmsg);
g_form.setValue('replace_with_field_this_is_run_against', ''); //clear out the field if value contains a character that does not match.
}
}
}
