Question: Is there a way to use the HttpPostedFileBase elements to find out if its a valid video file fomart (except of validating the extension). Im using aspnet mvc 3 by the way.

Answer: 
At every character between your start and stop words, you have to make sure that it doesn't match your negative or stop word. Like this (where I've included a little white space for readability):

```
^i ( (?!hate|pie) . )* pie

```

Here's a python program to test things.

```
import re

test = [ ('i sure like eating pie, but i love donuts', True),
         ('i sure like eating pie, but i hate donuts', True),
         ('i sure hate eating pie, but i like donuts', False) ]

rx = re.compile(r"^i ((?!hate|pie).)* pie", re.X)

for t,v in test:
    m = rx.match(t)
    print t, "pass" if bool(m) == v else "fail"

```
To match no `C` between `...A...B...`

Test in `python`:

```
$ python
>>> import re
>>> re.match(r'.*A(?!.*C.*B).*B', 'C A x B C')
<_sre.SRE_Match object at 0x94ab7c8>

```

So I get this regex:

```
.*\bi\b(?!.*hate.*pie).*pie

```