"The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is compiled with the default internal linkage size of 2. If you want to process regular expressions that are truly enormous, you can compile PCRE with an internal linkage size of 3 or 4 (see the README file in the source distribution and the pcrebuild documentation for details). In these cases the limit is substantially larger. However, the speed of execution will be slower."
This at least explain why changing the amount of memory available to PHP has no effect.
I did a small amount of googling to look for a php mechanism for detecting compiled pattern size but wasn't able to find anything. It would be worthwhile to discover the compiled size of the standard dokuwiki parser pattern and also the effect on that size of adding wordblocks, acronyms and plugins. i.e. is DokuWiki push close to the limit?
A solution for your particular problem may be to recompile PCRE with a higher linkage number.