Boost appears to support branch reset. So you could use something like:
^(?|<([A-Za-z0-9_]+)>|\(From ([A-Za-z0-9_]+)\):) MC_BOT ([a-z]+)( [a-zA-Z0-9 ]+)?$
^ ^ ^ ^ ^
| \ group 1 \ also group 1 \ g.2 \ group 3
|
\ branch reset