Replace named group in regex with value

Question

I want to use regular expression same way as string.Format. I will explain

I have:

string pattern = "^(?<PREFIX>abc_)(?<ID>[0-9])+(?<POSTFIX>_def)$";
string input = "abc_123_def";
Regex regex = new Regex(pattern, RegexOptions.IgnoreCase);
string replacement = "456";
Console.WriteLine(regex.Replace(input, string.Format("${{PREFIX}}{0}${{POSTFIX}}", replacement)));

This works, but i must provide "input" to regex.Replace. I do not want that. I want to use pattern for matching but also for creating strings same way as with string format, replacing named group "ID" with value. Is that possible?

I'm looking for something like:

string pattern = "^(?<PREFIX>abc_)(?<ID>[0-9])+(?<POSTFIX>_def)$";
string result = ReplaceWithFormat(pattern, "ID", 999);

Result will contain "abc_999_def". How to accomplish this?

Answer 1

No, it's not possible to use a regular expression without providing input. It has to have something to work with, the pattern can not add any data to the result, everything has to come from the input or the replacement.

Intead of using String.Format, you can use a look behind and a look ahead to specify the part between "abc_" and "_def", and replace it:

string result = Regex.Replace(input, @"(?<=abc_)\d+(?=_def)", "999");