Black and white binary code
However, in non-English regular expressions, this approach is not accurate unless a is the first lowercase letter and z is the last lowercase letter in the collation sequence of the language. The character class definition is based on the character set classification data. A base letter and all of the accented versions of the base constitute an equivalence class.
The current implementation does not support matching of Unicode composed and decomposed forms for performance reasons. The value can also be explicitly overridden by the runtime match option 'c' (case sensitive) or 'i' (case insensitive).