/usr/share/source-highlight/extreme_comment.lang is in libsource-highlight-common 3.1.7-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 | environment comment start '^[[:blank:]]*#if[[:blank:]]+0' begin
comment start '^[[:blank:]]*#(else|endif)' exit
end
include "cpp.lang"
|