The category "<unit_call_sign-gapsout_null-gapsin_null>" can be expressed as follows:

<unit_call_sign-gapsout_null-gapsin_null>

<any_letter> <digit>

<any_letter> <digit> <any_letter>

<any_letter> <digit> <any_letter> <digit>

<any_letter> <digit> <digit>

<dynamic_unit_call_sign>