In C#, how can I get the start/end indexes of all the replacements by the Regex.Replace() function

I have made a program to highlight the phrases in the input matched by a given Regex expression on the fly.

However, I want to highlight the replacements in the output panel too. To do this, I need to obtain the indexes and lengths found by Regex.Replace(). Unfortunately, it would seem C# doesn't give access to this data. Have I missed something?

I've thought about manually trying to figure out the indexes by accumulating sums given off from the MatchCollection produced by Regex.Matches(). But this is prone to error, and may not take into account the special $ symbol in the replace expression which could throw the figures off.

There must be a more elegant way.