refactor: simplify and document regexp

It took me a moment to understand what the given regexp is doing due to 2 reasons:
* AFAIK `@` is not a special char and doesn't need escaping
* Since we are matching beginning of the input we don't need the `g` modifier (?)
* Since we are matching letters we don't need the `i` modifier

Small change but should help with understanding of this code in the future.