Some help with rewrite_rules.php?

Any help with these problem I can't solve? Thanx in forward..

Problem:
I have file based url scheme and I am trying to change the names on rewrite_rules.php;
'forum' and 'topic' into 'text' and 'texts' in this rule:

'/^feed[\/_-]?(rss|atom)[\/_-]?(f|t)(orum|opic)[\/_-]?([0-9]+)[\/_-]?(\.xml?|\/)?$/i'                                    =>   
'extern.php?action=feed&$2id=$4&type=$1',

I tried this following but it doesn't work:
'/^feed[\/_-]?(rss|atom)[\/_-]?(t|t)(ext|exts)[\/_-]?([0-9]+)[\/_-]?(\.xml?|\/)?$/i'                                    =>   
'extern.php?action=feed&$2id=$4&type=$1',