table prefixing

i wanna make a table prefix code, normally i would just change all the querys, but since this site is already built and it's quite big i can't do every query again. i have a function named db_query(), it does my mysql querys and some error reporting, now i wanna add regex's (or something else if that would work better) to it to prefix every table name supplied to it. Can somebody help? this is what i tought: table names only come after FROM or in a table.field expression, maybe it helps??? Thanks already, Ruben