[bash] How to grep, excluding some patterns?

Just use awk, it's much simpler than grep in letting you clearly express compound conditions.

If you want to skip lines that contains both loom and gloom:

awk '/loom/ && !/gloom/{ print FILENAME, FNR, $0 }' ~/projects/**/trunk/src/**/*.@(h|cpp)

or if you want to print them:

awk '/(^|[^g])loom/{ print FILENAME, FNR, $0 }' ~/projects/**/trunk/src/**/*.@(h|cpp)

and if the reality is you just want lines where loom appears as a word by itself:

awk '/\<loom\>/{ print FILENAME, FNR, $0 }' ~/projects/**/trunk/src/**/*.@(h|cpp)