try this
editor = CKEDITOR.instances.fck; //fck is just my instance name you will need to replace that with yours
var edata = editor.getData();
var replaced_text = edata.replace("idontwant", "iwant this instead"); // you could also use a regex in the replace
editor.setData(replaced_text);
you may want to put that in a blur event or soemthing
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