Deriving the regular expression for C-style /**/ comments
I can think of four ways:


Define an automaton for the language you are interested in. Convert the regular expression to an automaton (using Brzozowski's derivatives). Check that both automata accept the same language (determinize and minimize or use a bisimulation argument).
Write loads of test cases and apply your regular expression to them.
Convert the automaton defined in point 1 to a regular expression, using standard techniques.
A combination of the above.