In some common implementations if $LANG is set to certain values, it will fail to match some ASCII letters. This is because not all latin character using languages put Z last in the alphabet.

Try this (you probably need to enable and generate the locale first)

    echo y | LANG=lv_LV.UTF-8 grep '[a-z]'

Locales in general should be considered a "trap", just look at Windows CSV separator handling, etc.

It depends on its use, ultimately, but if your goal is to find a string of letters (a common use IMO), you'll want to use something like \p{L} to ensure you don't miss non-ASCII characters.

eta: fixed regex, I had typed \L, shared from my faulty memory.

[A-z] though is a fun one though as it includes a few extra symbols between upper and lowercase.