This:
(.*?)\s*(.*?)\s*
can be problematic. Since LABEL and PARAMETER must not contain spaces, try replacing that portion with something like:
([^\s]+)\s+([^\s]+)?

I'd also suggest that you change *? to simply *, since * denotes 0 or more occurances.

Probably the biggest problem is the:
(.+?)
which occurs just before:
<%end_\1%>
and matches to the end of the string, which then causes the regex engine to backtrack each time it cannot find each "end" tag.

You may also want to take a look at the book "Mastering Regular Expressions" by Jeffrey E. Friedl. See Mastering Regular Expressions for a review.