validation pattern for text field (number only)

Hi, Im trying to create a field that users can only enter number. If they enter text, an error message should pop up. I can't seem to get it to work although i got the error message to show when users enter text. However, im not sure what i need to put in the "validation pattern" box. Should it be 9 or 99999 or 999999999. None of those worked, as i have tried. If someone has any idea. Please help. Thanks.