The Arduino IDE and its underlying compiler, recognizes strings in the C sense, which are null-terminated character arrays. However, because the underlying compiler also supports C++, you can also use the String class. (Note the uppercase 'S'.) In most cases, I think most prefer the character array approach to using string data in C.
There are numerous string processing functions available to you from the C library. Most of these begin with "str" (e.g., strcmp(), strcat(), strtok(), etc.) There are also a number of preprocessor macros that look like function but are actually macro, such as toupper(), tolower(), isalpha(), etc. If you take a little time and do some Google searching, you'll likely find what you need. A redim is another animal, as there is no underlying op system to do garbage collection for you.