The PCRE library supports recursion using two different methods: one uses normal stack allocation and the other allocates memory on the fly as needed.
The reason your script is crashing the PCRE library is due to heavy recursion resulting in a stack allocation error. This is most likely due to an inefficient regular expression. I tested your script using memory-based recursion in PCRE and it completed without crashing, however the result from $feedtextconvert() was not correct.
In a previous version of mIRC, I changed PCRE to use memory based recursion since users were reporting the same issue as you. The change solved the heavy recursion crash issue, however it also resulted in far slower expression parsing, which some users reported
. So I changed it back to stack allocation in a subsequent version of mIRC.
Update: I have made a change to PCRE that limits the recursion depth and causes the parsing to stop at that point. Since it is stack-based, a recursion depth of about 2000 seems to be a safe maximum and prevents mIRC from crashing.