char * strcasestr(const char *, const char *) __ATTR_PURE__
The strcasestr() function finds the first occurrence of the substring s2 in the string s1. This is like strstr(), except that it ignores case of alphabetic symbols in searching for the substring. (Glibc, GNU extension.)
The strcasestr() function returns a pointer to the beginning of the substring, or NULL if the substring is not found. If s2 points to a string of zero length, the function returns s1.