I am sure that has been asked before, but I cannot find the appropriate question(s).

Being new to [C#'s Regex](https://learn.microsoft.com/en-us/dotnet/api/system.text.regularexpressions.regex?view=netframework-4.8), I want to mimic what is possible e.g. with `sed` and `awk` where I would write `s/_(20[0-9]{2})[.0-9]{1}/\1/g` in order to find obtain a 4-digit year number after 2000 which is has an underscore as prefix and a number or a dot afterwards. The `\1` refers to the value within brackets.

**Example:** Both files `fx_201902.csv` or `fx_2019.csv` should give me back `myYear=2019`. I was not successful with:

```cs
string myYear = Regex.Replace(Path.GetFileName(x), @"_20([0-9]{2})[.0-9]{1}", "\1")

```

How do I have to escape? Or is this kind of replacement not possible? If so, how would I do that?

**Edit:** My issue how to do the `/1` in C#, in other words how to extract a regex-variable. Please forgive me my typos in the original post - I am trying the new SO app and I submitted earlier than intended.