# Matches C/C++ #if/#ifdef/#ifndef .. #endif
(?m)(?<!//)\#\s{0,}(?=if)   #  As long as we're not after comments, Match the , followed by
(?<If>if[^\s]+)             # Match <If> (and the rest of the word)
(?<Condition>.+?$)          # the <Condition> is anything until the end of the line
# Now things get tricky.  Because ifdefs can nest, we need a balancing group
(?>
  [^\#]+                    # Any non-preprocessor character matches, and doesn't change the balance
  |
  (?<!//)\#if.+?$(?<Depth>) # An if Increases the <Depth>
  |
  (?<!//)\#endif(?<-Depth>) # An EndIf Decreases the Depth
  |
  \#                        # Match any remaining 
)*(?(Depth)(?!))            # Match Until EndIf is balanced
(?<!//)\#endif              # Match the endIf