add_filter *not* on the_excerpt

Hello!

I am working on a plugin that replaces [sometag] with some code. I want it to apply to every page and single post. But I do *not* want it to apply on the_excerpt (which is autogenerated).

I tried to apply my function to ‘the_content’ and wrap it with
if(!is_single() || !is_page())
But unfortunatelly those functions return false in any case when they are called inside my plugin: on the excerpt as well as on a page/single-post.

Does someone have an idea how to solve this?
Maybe I have to do it manually an wrap all my the_content-calls inside my template?

Regards