A local variable "highlight_regex" has been added in buffers.
It is recommended to use script buffer_autoset.py to set it, so that it's saved and set at any time when buffer is created.
However, it's still possible to set it manually (for testing) with command:
/buffer set highlight_regex XXX.
For example, to highlight all messages on server freenode, channel #test:
/autosetbuffer add irc.freenode.#test highlight_regex .*