This regular expression has several flaws:
1) looks to my that it has too many groups: (...)
2) too many * that lead to an exhautiive search (n-square of the document length?)
3) if it is multiline, the . will not match the end-of-line
4) there must be some non-greedy repetition, otherwise, you will finally match all from the first to the last entry at once (what is problably not what you want if you have more than one Car element.
For the simple IsMatch() call, the following is sufficient:
<Car>[\s\S]*?</Car>
Do I miss something?
Cheers
Andi