If you check "Match case" it should work. To make "Match case" available,
do not check "Use Wildcards," "Sounds like," or "Find all word forms."

--
Greg Maxey/Word MVP
See:
http://gregmaxey.mvps.org/word_tips.htm
For some helpful tips using Word.

Rasoul Khoshravan Azar wrote:
I wan to replace the abbrevation CAS (capital) in y file with
compressed air system (all small letter).
When I use find/replace command, word do so but it appears in capital.
(COMPRESSED AIR SYSTEM). There is an option inside replace which says
recognize capitals and small letters but it is grayed out. How can I
trun on this option?