Someone asked privately if the % in [^%>] was required. That is a good question, so I decided to answer it in public.

Without that %, we get: 

    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

[download]
which will match as follows: 
    "<% %%> %>"
    "<%" 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

[download]
so we've matched the whole string when we should have only matched the first part, "<% %%>".

By leaving the % out of [^%>], we've allowed the regex to back-track and match the first character of our delimiter (%) as the tail end of %+[^>].

But I now realize that my regex is also broken because it will never match: 

    "<% %%>"

[download]
at all. I'm tempted to fix it with: 
    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

[download]
but that seems wrong. Think...

Bah, I'm hours later for bed already. Serves my right for "showing off" "unrolling the loop" when I've seen *so many* *really good* regex slingers get this wrong more than once. (:

Unlike the last time I saw this happen, these nodes will *not* be updated to hide the mistakes I've made (that last time the updates were flying really fast and I was extremely frustrated by not being able to learn from the repeated mistakes).

                - tye

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

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