How to trigger a macro by certain string appearing on the screen

There is an easy way to trigger a macro when one types a string.
Is it possible to trigger a macro when the string appears on the screen (I use dictation software)?


That would require scanning the screen continuously which would be prohibitively expensive. Also, it would depend on the appearance of the string, which would depend on the font and size of the text.

Ok, thanks for the clarification.