Regex Troubles

I modified the data you provided so that it has just a linefeed within each set of invoices as you describe and the code I provided still processed the data correctly. So I think you don't need to modify the data before using the code I provided. The output data will have the linefeeds in the Invoice_Details but it should work. What you could do is after processing the data into a DataTable using the code I provided, you can use String.Split to split the invoices at the linefeeds and then store the invoices in a separate DataTable. Or you could use a regular expression to parse the invoices.

That data is in typical CSV format, except the linefeed is a minor problem. If it is possible to convert the linefeed within each set of invoices to something else, then you can read the data into a DataTable using about four lines of code, excluding the code for the connection string and SQL and the path and filename. It can be that easy; you don't need to mess with a regular expression.
 
I am uploading a zip file with sample source code and a txt file with your data modified. I modified your data so that the linefeeds are "\r\n" instead. This shows you that it is easy to read the data into a DataTable, except you probably need to convert the linefeeds to something. The conversion is not as simple as converting all linefeeds, since you don't want to convert the linefeeds that are followed by a carriage return.

Try testing your regular expression on this link

http://www.regexlib.com/RETester.aspx

It'll give you exactly what you want....