Thanks for your bug report. I have been able to reproduce this issue. This is due to a change in the very first Unicode version of mIRC, which required introducing UTF-8/UTF-16 conversions throughout the code. I have updated the next beta so that the 'w' option will allow unlimited string length matching.
However, this cannot be easily changed for regex as the string length limit is something that is imposed throughout the regex routines. Allowing unlimited string lengths in the regex routines would require a significant rewrite. This has been added to my to-do list.
