Hm, that is an issue... I see two approaches here: 

1. Fully compatible with Syntastic options; the value of a Syntastic option can be used as the value of the corresponding YCM option.
2. Conceptually compatible with Syntastic options: anything you can configure with Syntastic you should be able to configure with YCM, possibly using a slightly different option syntax.

We should strive for 1. when we can and 2. where 1. makes less sense.

Going with 1. here would cause both perf pain and a usability hit since Vim regexes are less widely known. This option also doesn't need regexes, globs are just fine. Using globs would also be more consistent with other, similar YCM options.

To tell you the truth, the more I think about this option the more I'm realizing that YCM probably shouldn't have it. With Syntastic, the only feature you get is diagnostics. Turning that off for certain files makes sense. In YCM, the diagnostics are only a sub-feature of YCM compiling your files with libclang to provide semantic completion. If you have errors in your file and are trying to use semantic completion, it probably won't work (because of the errors, so you should be notified when we have them). And if you don't want semantic completion for that file, then we need something other than this option.

I think we need a different option, which is "turn off the semantic engine (diagnostics included) for these files."