[Boost-users] regex lib - using replace

Hi, I need one regular expression and one format string to do the following 1) if text starts with s1, replace s1 with r1 2) else, if text starts with s2, replace s2 with r2 3) else, if text starts with s3, replace s2 with r3 4) else, prepend r3 to text While I managed to work out the first three using an expression like (^s1)|(^s2)|(^s3) and the format as 1?r1:2?r2:3?r3: I have no idea how to include the fourth case into my expression. Any suggestion? Thank you in advance, Lucian