replace substring, wildchards

 VS 2008, .NET 3.5, C#, webapplication
Hi
I need to replace a substring from a string.
I know the beginning of the substring to replace, its length but not the characters of the ending part.
For this reason I cannot use string.replace in the usual way.
Is there something like a wildchard character, to use like string.replace("this is red ###", "it is green"); ?
### symbolizes the three characters I do not know.
If this is not possible, what is the easiest way to replace the substring?
Thanks for your hint.
Patrick
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