Hello.

Use sed in a do shell script. short example (on the fly).

  set mytext to "sise" & LineFeed
  set fixed to do shell script "sed -n 's/size/sise/g' <<<" & mytext

I have written a script, for letting you first copy your terminal commandline, when you have figured it all out, so that you can paste it into a do shell script, it can be found here <http://macscripter.net/viewtopic.php?pid=163300#p163300>; And Macscripter.net is a great forum for AppleScript questions.