That expression is causing problems (in terms of crashing mIRC) here, too, after we played a bit with
tokenize instead of variable assignment.
The regular expression is incomplete anyway (it does not match an <img> code), so I tried to fix it. After some more testing, the following line:
//echo -ag $regsubex(<img src="test.png" alt="test" />,/<img([^>]*)alt="([^"]+)"((\s|\S|=|")*)(\s)?(\/)?>/g,\2)
... echoes "test" correctly, so, at least, the "fixed" expression
matches. Using it in the particular
%feedconverted line still makes it crash mIRC, so (obviously) it does not even matter if the expression matches or not.
(I wonder why the stack overflow only applies to some of the rules and does not happen when commenting out some.)