Here's an alternative, in case yours doesn't work: 
# Read these comments from the bottom up.
/
   (
      (?:
         (?! \s{2} )  #    which aren't the start of 2 spaces.
         .            #    characters
      )*              # zero or more
   )                  # Capture
/sx

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In reply to Re^4: how to find what's not there with a regex? by ikegami
in thread how to find what's not there with a regex? by samizdat

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