How can I run lots of RegExes (to find matches) in big strings without causing LOH fragmentation?

It's .NET Framework 4.0 so I'm using `StringBuilder` so it's not in the LOH however as soon as I need to run a RegEx on it I have to call `StringBuilder.ToString()` which means it'll be in the LOH.

Is there any solution to this problem? It's virtually impossible to have a long running application that deals with big strings and RegExes like this. 

**An Idea to Solve this problem:**

While thinking about this problem, I think I found a dirty solution. 

At a given time I only have 5 strings and these 5 strings (bigger than 85KB) will be passed to `RegEx.Match`.

Since the fragmentation occurs because new objects won't fit to empty spaces in LOH, this should solve the problem:

1. `PadRight` all strings to a max. accepted size, let's say 1024KB (I might need to do this with `StringBuider`)
2. By doing so all new strings will fit to already emptied memory as previous string is already out of scope
3. There won't be any fragmentation because object size is always same hence I'll only allocate 1024\*5 at a given time, and these space in LOH will be shared between these strings.

I suppose the biggest problem with this design what happens if other big objects allocate this location in LOH which would cause application to allocate lots of 1024 KB strings maybe with an even worse fragmentation. `fixed` statement might help however how can I send a fixed string to RegEx without actually create a new string which is not located in a fixed memory address? 

Any ideas about this theory? (Unfortunately I can't reproduce the problem easily, I'm generally trying to use a memory profiler to observe the changes and not sure what kind of isolated test case I can write for this)