m[
      <%        # Opening delimiter.
      (?:       # Match stuff that isn't a closing delim:
        [^%]+     # Things that can't start one.
      | %+[^>]    # Might start one but isn't one.
      )*        # As many non-closing-delims as you like.
      %>        # Closing delimiter.
    ]x

##</code><code>##

    "<% %%> %>"
    "<%" 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

##</code><code>##

    "<% %%>"

##</code><code>##

    m[
      <%        # Opening delimiter.
      (?:       # Match stuff that isn't a closing delim:
        [^%]+     # Things that can't start one.
      | %+[^%>]   # Might start one but isn't one.
      )*        # As many non-closing-delims as you like.
      %*        # PUNT!
      %>        # Closing delimiter.
    ]x