Imagine as a finite state machine ....
   input is received by getchar()

   State is ----
      buffer:  up to 10 characters that could be output
      count:   number of characters in buffer
      badline: set to 1 if bad characters found

Before entering loop (and when beginning to process a new line)
   set badline to 0
   set count to 0

Program is a single while loop that use getchar() to read a character
   Performs test on the character
      if whitespace and not '\n':
         Do nothing
      if alphabetic or digit:
         if (count >= 10 or badline==1)
            set badline to 1
         else
            if character is alphabetic
               translate to range of '2' to '9'
            add character to buffer at position count
            increment count
      if '\n' or EOF
         if badline == 1 or count != 7 and count != 10
            print error message
         else
            print buffer in nnn-mmm-pppp format
         if character is "EOF"
            exit
         set count to 0
         set badline to 0
      otherwise
         set badline to 1