Word Wrapping with Regular Expressions

Question

EDIT FOR CLARITY - I know there are ways to do this in multiple steps, or using LINQ or vanilla C# string manipulation. The reason I am using a single regex call, is because I wanted practice with complex regex patterns. - END EDIT

I am trying to write a single regular expression that will perform word wrapping. It's extremely close to the desired output, but I can't quite get it to work.

Regex.Replace(text, @"(?<=^|\G)(.{1,20}(\s|$))", "$1\r\n", RegexOptions.Multiline)

This is correctly wrapping words for lines that are too long, but it's adding a line break when there already is one.

Input

"This string is really long. There are a lot of words in it.\r\nHere's another line in the string that's also very long."

Expected Output

"This string is \r\nreally long. There \r\nare a lot of words \r\nin it.\r\nHere's another line \r\nin the string that's \r\nalso very long."

Actual Output

"This string is \r\nreally long. There \r\nare a lot of words \r\nin it.\r\n\r\nHere's another line \r\nin the string that's \r\nalso very long.\r\n"

Note the double "\r\n" between sentences where the input already had a line break and the extra "\r\n" that was put at the end.

Perhaps there's a way to conditionally apply different replacement patterns? I.E. If the match ends in "\r\n", use replace pattern "$1", otherwise, use replace pattern "$1\r\n".

Here's a link to a similar question for wrapping a string with no white space that I used as a starting point. Regular expression to find unbroken text and insert space