Solution.

/filter and $hregex (and probably others) don't interpret flags that appear outside the delimiters. For whatever reason, mIRC simply strips the delimiters and any flags that follow. eg: /i

The same flags can be used inside the expression though, placed before the pattern you wish for it to effect. This even gives you more control, allowing you to make just part of the expression case insensitive (i *think*, haven't tested it and don't know how to close/terminate it either.)

/expression/i == /(?i)expression/

- Raccoon

Well. At least I won lunch.
Good philosophy, see good in bad, I like!