"<% %%> %>"
    "<%" matches <%
      " " matches [^%]+ so (?: ... )* has matched once
       "%%" matches %+
         ">" fails on [^>] so we back-track
       "%" now matches %+
        "%" matches [^>] so (?: ... )* has matched twice
         "> " matches [^%]+ so (?: ... )* has matched 3 times
           "%>" matches %> so regex finishes