regex

I am trying to convert ms access sql to postgresql using php. I have a sql statement in the form ;- $sql = SELECT DISTINCT [Table Name].[Column.Name], [Table Name 1].[Column Name 2] etc. what I want to end up with is $sql = SELECT DISTINCT table_name.column_name, table_name_1.column_name_2, ........ I have managed to get the caps to lower but I cant work out how to put the _ in place of spaces if the spaces are between [ ]. I either end up with S_E_L_E_C ..... or SELECT_DISTINCT_ etc... . Naturally I have only used part of sql statement and table, column names have been changed. (Think the one I'm trying on is 2000+ characters. So its not a case of set number of words/numbers between [] it could be 2 or it could be 4 etc) Anyone workout what I am talking about and can help would be appreciated. Peter