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

<unit_call_sign>

<any_letter> <digit>

<any_letter> <digit> <any_letter>

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

<any_letter> <digit> <digit>

battalion <digit-not_zero>

battalion <digit-not_zero> <digit>

battalion <digit-not_zero> <digit> <digit>

battalion <digit-not_zero> <digit> <digit> <digit>