Specifies details about changing the case of letters. It may contain these words, separated by a comma: internal Use internal case mapping functions, the current locale does not change the case mapping. This only matters when 'encoding' is a Unicode encoding, "latin1" or "iso-8859-15". When "internal" is omitted, the towupper() and towlower() system library functions are used when available. keepascii For the ASCII characters (0x00 to 0x7f) use the US case mapping, the current locale is not effective. This probably only matters for Turkish.

(default: "internal,keepascii")