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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In reply to Re: assertions help by ikegami
in thread assertions help by DavidFerrington

Title:
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