Simple Line Editor: How to parsing commands?

I am supposed to write a simple line editor. The bit I have been stuck at for a few days is "parsing commands". The users enters a line range (12,27) and then a command (c), e.g., 12,27c or 14,1532c (all whitespaces are ignored). The command line can have missing parts, e.g., 27c or 27 or , or symbols "." (Current Line in Buffer) and "$" (Last Line in Buffer), for example: .,$c. I have to extract the info entered by user and then use default values for the missing bits and bobs. My problem is: I cannot find a way to tokenise such a string when there are no delimiters. I would very much appreciate a nudge in the right direction! Another way that I thought of is to test the input for int and get the int if it is there, if not read a character ("," or "c"), and so on ... I am not looking for a solution so much so as I am looking for an idea to help put me on the correct path.