C# regex replace a name group with another value

I have a document of text that I use a regex to look for a specific line.  Within this specific line, there are named groups.  For one of these groups, I need to replace the value with one that is supplied by the user.  I cannot do a simple find/replace because the replacement is only applicable in the line of text matched by the regex.  I am working with EDI files.  The example I'm using below is simply an example, not a properly formatted EDI document.  The "edi" variable is my source document.

const string lastNamePattern = @"(?<NM1>NM1\*IL\*1\*(?<LastName>[a-zA-Z0-9]{0,60})\*[a-zA-Z0-9]{0,35}\*\*\*\*MI\*[a-zA-Z0-9]{2,80}~)";

const string edi = "ISA*PERRY~NM1*IL*1*PERRY*CHRISTOPHER****MI*1234567890A~IEA~";

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I want to replace the named group "LastName" within the "NM1" group with another value.  I need to return the whole edi string with the replaced value.  Any help would be appreciated!
cperry-scaAsked:
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Terry WoodsIT GuruCommented:
I'm a PHP programmer, but I can say it should be pretty easy to do it like this (code generated by myregextester.com):
using System;
using System.Text.RegularExpressions;
namespace myapp
{
  class Class1
    {
      static void Main(string[] args)
        {
          String sourcestring = "source string to match with pattern";
          String matchpattern = @"(NM1\*IL\*1\*)(?<LastName>[a-zA-Z0-9]{0,60})(\*[a-zA-Z0-9]{0,35}\*\*\*\*MI\*[a-zA-Z0-9]{2,80}~)";
          String replacementpattern = @"\1NewLastName\3";
          Console.WriteLine(Regex.Replace(sourcestring,matchpattern,replacementpattern));
        }
    }
}

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Result:
ISA*PERRY~NM1*IL*1*NewLastName*CHRISTOPHER****MI*1234567890A~IEA~

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You should be able to play with it here: http://www.myregextester.com/?r=c9da41c5
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Terry WoodsIT GuruCommented:
I've wrapped the text before the last name in their own capturing brackets, and the same for the text after the last name. The replacement pattern puts that captured text back into the result, along with the value "NewLastName" which you can change to whatever you need.
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