Regex- Does case insensitivity only apply to letters (upper and lower case)?

Question

(Sorry I know the question sounds ridiculous) and if so which of the following is preferred:

[a-zA-Z]
[a-z]i
[A-Z]i

Answer 1

If the regex engine supports Unicode and the appropriate option is passed (e.g. in PCRE, the u modifier), then case insensitivity is also applied to Unicode character sequences where appropriate.