(?(?!PATTERN).)*

##</code><code>##

(?(?!^#\s*else\b).)*

##</code><code>##

#ifdef X
...a...
#  ifdef Y
...b...
#  else
...c...
#  endif
...d...
#else
...e...
#  ifdef Y
...f...
#  else
...g...
#  endif
...h...
#endif