A simple example:
Regex.Replace("12345678910999999999", @"(\d{3})(\d{3})(\d{3})(\d{2})", "$1-$2-$3 $4")
This outputs to:
123-456-789 10999999999
But why? I have specifically set the group index i need. And that group index contains the exact value (checked in debugger).
Here is a fiddle: https://dotnetfiddle.net/dkAPx3
Match the rest of the string with .* to truncate it:
Regex.Replace("12345678910999999999", @"^(\d{3})(\d{3})(\d{3})(\d{2}).*", "$1-$2-$3 $4")
I'd also add ^ at the start to match the beginning of the string.
See the .NET regex demo.
Your regex has matched and replaced only "first" part of string, add .* to the end of the pattern:
Regex.Replace("12345678910999999999", @"(\d{3})(\d{3})(\d{3})(\d{2}).*", "$1-$2-$3 $4"); // results in "123-456-789 10"