If you don't care if the words in the source string are separated by one or multiple blanks and you don't need to maintain this then use Option 1.
If you need to maintain the number of separating blanks then consider Option 2.
data have;
infile datalines delimiter=',' dsd truncover;
input string :$40. newstring :$40.;
datalines;
F03 J47 S528 X590,F03 J47 XXXX XXXX
E872 R263 S720 X590,E872 XXXX XXXX XXXX
S720 X590,XXXX XXXX
S150 V789,XXXX V789
;
run;
data want(drop=_:);
set have;
/* option 1 */
length newstring_2 $40 _term $10;
do _i=1 to countw(string,' ');
_term=scan(string,_i,' ');
if findc(_term,'rsx','i') then _term='XXXX';
newstring_2=catx(' ',newstring_2,_term);
end;
/* option 2 */
length newstring_3 $40.;
newstring_3=prxchange('s/\b[^ ]*[rsx][^ ]*\b/XXXX/i',-1,strip(newstring));
run;
Based on your sample data I'm also converting words to XXXX that contain the letter R.
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