The c-strcasestr module contains a case-insensitive string search
function operating on single-byte character strings, that operate as if the
locale encoding was ASCII.
(The "C" locale on many systems has the locale encoding "ASCII".)
The function is:
extern char *c_strcasestr (const char *haystack, const char *needle);
For case conversion here, only ASCII characters are considered to be upper case or lower case.
Note: The function strcasestr from <string.h> supports only
unibyte locales;
for multibyte locales, you need the function mbscasestr.