How do I split this string?

This is a text file that I am reading into my program. I need to read each line and into a string and then split the string so that I can store for example, the PROG: label into a label variable and START into a symbol variable and #0 into the operand variable. The goal at the end is to store this information into a binary search tree. The problem here is that some of the lines do not contain any label therefore I am not sure how to solve that problem.

PROG:   START   #0
ONE:    FIX
    COMPR   S,T
    LDL #10
    STB TWO,X
    SVC #5
TWO:    NORM
    END ONE