The simplest solution is to just remove the quantifier, and add a negative lookahead. Then the regex becomes: 

    m[
      <%           # Opening delimiter.
      (?:  [^%]+   # Things that can't start one.
         | % (?!>) # % that aren't part of a closer
      )*           # As many non-closing-delims as you like.
      %>           # Closing delimiter.
    ]x

[download]

Backtracking no longer needed. (-:

In reply to Re: Re^2: A NOT in regular expressions (why [^%>]?) by jryan
in thread A NOT in regular expressions by Anonymous Monk

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