char *strcasestr(const char *s1, const char *s2);
strcasestr locates the first occurrence in the string pointed to by s1 of the sequence of characters (excluding the terminating null character) in the string pointed to by s2 without regard to character case.
strcasestr returns a pointer to the located string, or a null pointer if the string is not found. If s2 points to a string with zero length, strcasestr returns s1.
strcasestr is an extension commonly found in Linux and BSD C libraries.