Red Kalinka Forum

Hello everyone, newbie here.
I read that the nominative plural of feminine nouns ending with -а is formed putting -ы.
If it's true, why is the plural form of собака "собаки" and not "собакы"? Is this word an exception?

I think it is because of the "seven letter rule" in Russian:

After consonants г, к, х, ж, ш, щ, ч do not write the letter ы. Whenever an [ы] or [и] sound
follows one of these letters, it is spelled и.