PHP regular expressions are supplied by a library called pcre (or libprce). An uprev in that library (to 6.7 iirc) has caused this problem to start happening - although not normally in the lexer.
You can try your page on my site at
http://wiki.jalakai.co.uk/dokuwiki/ to see if it works there.
Also, you may want to investigate whether or not you can upgrade to v7 of the library. I don't know if that has any impact, however the changelog does include a couple of changes concerned with memory usage computations.