[boost] [regex] Change format of data in file

Hello all, I have file output.txt, in file we can find something like: 01/01/2015 15:00:00 2222.2222 2222.2222 2222.2222 2222.2222 04/04/2015 15:00:00 2222.2222 and i would like to change format from DD/MM/RRRR HH:MM:SS to RRRR-MM-DD HH:MM:SS.000 using boost regex. But I have no clue how should I type pattern and replace them with another. Anyone have knowledge about boost regex and could help me ? Best regards