I think it matches any string that contains an "a".  At least that's what my limited testing shows. 4043 data _null_; 4044 input line $20.; 4045 if _n_ eq 1 then rx=prxparse("/.*a.*/"); 4046 retain rx; 4047 match = prxmatch(rx,strip(line)); 4048 put (_all_)(=); 4049 cards; line=hello rx=1 match=0 line=helloah rx=1 match=1 line=zzz rx=1 match=0 line= rx=1 match=0 line=ahhello rx=1 match=1 line=678ab rx=1 match=1 line=a rx=1 match=1 line=ta rx=1 match=1 line=u rx=1 match=0 ... View more

You appear to have a strange set of bracketing there.  If you put them on separate lines, you will see the first ( is closed before the last cow record, hence it will work fine to that point.  It is a good example of why Good Programming Practice is important, i.e. consistent casing, spacing, use new rows to split things up etc.: ('Beef'      (           ('Cattle'                 ('EB' CATTLE_EB_N CATTLE_EB_P)                 ('SB'  CATTLE_SB_N CATTLE_SB_P)           )      )                 ('Cow'           ('EB' CATTLE_EB_N CATTLE_EB_P)           ('SB'  CATTLE_SB_N CATTLE_SB_P)      ) )      ('Cow'           ('EB' CATTLE_EB_N CATTLE_EB_P)           ('SB'  CATTLE_SB_N CATTLE_SB_P)      ) ) ... View more