How do I find the offset of a matching string using RE2?

RE2 is a modern regular expression engine available from Google. I want to use RE2 in a program that is currently using gnuregex. The problem I have relates to finding out what matched. What RE2 returns is the string that matched. I need to know the offset of what matched. My current plan is to take what RE2 returns and then use a find on the C++ string. But this seems wasteful. I've gone through the RE2 manual and can't figure out how to do it. Any ideas?