SAS to keep only valid characters in a String

/* These are the only valid characters */
%let validchars = 'a' 'b' '1' '2' '_' '$';


data validchars;
string = "abc123_*";
str_len = length(string );


do while( str_len > 0);
if substr(string, str_len,1) not in (&validchars )then substr(string,str_len,1) = " ";
str_len = str_len -1;
end;
string = compbl(string);
/*Removing multiple blanks */
run;
proc print;
run;