If you are a zealot of FOSS (Free and Open Source Software) like you claim and you use either Linux (or as I've taken to calling it, GNU/Linux because Linux by itself is just the kernel while the GNU Project provides the core tools) or another similar free operating system such as FreeBSD or OpenBSD, then might I recommend that you search through the source code using the tool `grep'. This tool is considered a standard in the Unix and Unix-like world, and thusly is usually part of the core utilities meaning it should be part of the base system without having to install a package.
For example, in this situation you would be looking to use `grep' with the `-nri' flags, to get the line number (-n), do it recursively (-r) and do it case-insensitively (-i). I recommend checking out the manual pages (this is done with the `man' command).
HTH,
Cheers,
Byte