Regular Expressions-replacing part of a line in text file

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C#

Hi Guys

I need to edit part of a specific line in a text file.

The line in the text file I want to edit is:
login = '. login username password\r\n'

I want to replace the username and password with the ones entered in my form text boxes.

The current username and password fields in the above line in the text file could be anything.

How can I replace the above line with my form textbox username and password strings without knowing exactly what the current username and password are??

I have used the code below but this only replaces strings that I specify and that I know :(

public static bool Replace(ref string file, ref string searchFor, ref string replaceWith)
        {
            try
            {   
                StreamReader reader = new StreamReader(file);
                string contents = reader.ReadToEnd();
                reader.Close();
                reader.Dispose();
                contents = Regex.Replace(contents, searchFor, replaceWith);
                StreamWriter writer = new StreamWriter(file);
                writer.Write(contents);
                writer.Close();
                writer.Dispose();
                return true;
            }

any help would be appreciated. Many thanks!