formatting should be 7 digits + a space + 4 digits = 11 digits with a space in the middle. e.g. 1234567 1234

You can use the like() function. Something like this seems to meet your requirements:

fn!like(
  ri!input,
  "[1-9][1-9][1-9][1-9][1-9][1-9][1-9] [1-9][1-9][1-9][1-9]"
)

(note that there is a space between the first group of seven [1-9] and the last group of four [1-9]. Also note that if you want to allow a zero in either of your groupings of digits you can adjust the relevant [1-9] to [0-9] as required)

...where ri!input is a text field.