Implement full case mapping according to SpecialCasing.txt and
also full case folding according to CaseFolding.txt (F). There
are a number of caveats:
* Only language-agnostic and unconditional full case mapping
is implemented. The only language-agnostic conditional case
mapping rule relates to Greek sigma in final position
(Final_Sigma). Correctly handling this requires both arbitrary
lookahead and lookbehind, which would require some larger
changes to how the case mapping is implemented. This is a
possible future extension.
* The only language-specific handling that is implemented is
for Turkish dotted/undotted Is, if the ISO-8859-9 encoding
is used. This matches the previous behavior and makes sure
that no codepoints not supported by the encoding are
produced. A future extension would be to also handle the
Turkish mappings specified by SpecialCasing.txt based on
the mbfl internal language.
* Full case folding is implemented, but case-insensitive mb_*
operations continue to use simple case folding. The reason is
that full case folding of the haystack string may change the
position at which a match occurred. This would have to be
mapped back into the position in the original string.
* mb_convert_case() exposes both the full and the simple case
mapping / folding, where full is the default. The constants
are:
* MB_CASE_LOWER (used by mb_strtolower)
* MB_CASE_UPPER (used by mb_strtolower)
* MB_CASE_TITLE
* MB_CASE_FOLD
* MB_CASE_LOWER_SIMPLE
* MB_CASE_UPPER_SIMPLE
* MB_CASE_TITLE_SIMPLE
* MB_CASE_FOLD_SIMPLE (used by case-insensitive operations)
