The following can usually be used to skip over a section that doesn't contain something that matches PATTERN:

(?(?!PATTERN).)*
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Sounds like you want

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

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