Selecting always third and forth letter in a word and change their character style

Hello @Martina zstudio 

and the other grep:

.{2}(?=.\t|.$)

For use in GREP Style in paragraph style* ou in Find/Change.

* This is the best way because the text is automatically formatted wherever the paragraph style is applied. However this will always be an indirect formation, for PDF and the like it will be kept. For copying and pasting between applications, it may not work at the destination.

second and third letters or numbers of every word

GREP Style:

\b(\w\w)\K\w\w

All roads lead to Rome

If each word is in its own text frame, this should do the job:

var st = app.activeDocument.stories;

for(i=0; i<st.length; i++) {
	if(st[i].characters.length >= 3 ) {
		st[i].characters[2].fontStyle = st[i].characters[3].fontStyle = "Bold";
		st[i].characters[2].pointSize = st[i].characters[3].pointSize = 14;
	}
}

Before:

After: