def map_expr(db, expr, mapping, in_cols_found=None):

    '''Replaces output columns with input columns in an expression.

    @param in_cols_found If set, will be filled in with the expr's (input) cols

    '''

    for out, in_ in mapping.iteritems():

        orig_expr = expr

        out = sql_gen.to_name_only_col(out)

        in_str = sql_gen.to_name_only_col(sql_gen.remove_col_rename(in_)

            ).to_str(db)

        # Replace out both with and without quotes

        expr = expr.replace(out.to_str(db), in_str)

        expr = re.sub(r'\b'+out.name+r'\b', in_str, expr)

        if in_cols_found != None and expr != orig_expr: # replaced something

            in_cols_found.append(in_)

    return expr