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

<unit_call_sign>

<any_letter> <digit> <any_letter>

<rwa_name>

<ship_name>

battalion <digit-not_zero>

battalion <digit-not_zero> <digit>

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

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

the <rwa_name>

the <ship_name>

x <rwa_name>

x <ship_name>