If you need to use a regex, you can do this:
Regex.Replace(input, "(?<=^.{10}).*", "...");
This replaces any text after the tenth character with three dots.
The (?<=expr)
is a lookbehind. It means that expr
must be matched (but not consumed) in order for the rest of the match to be successful. If there are fewer than ten characters in the input, no replacement is performed.
Here is a demo on ideone.