I know everyones going to kill me... but I am just putting the finishing touches on an upgraded version that is substantially faster (not that this is slow by any means) and eliminates the query, include file,the array walks, and the inline replacments. I have re-coded it to build these up in the admincp when ammendments are made. It then stores it in the datastore.
I am also putting in the option to add an additional leading space to eliminate breaking existing URLs. The flip side to this is that the key word must have a space infront of it to be a match.
Ill post more when I have the update.
Give me an hour (PS- the upgrading process should be a snap so dont anyone get fidgety with me

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