>perl -wMstrict -le
"my @tests = (
   'RRULE:FREQ=WEEKLY;BYDAY=TU,TH;UNTIL=20110429T000000;WKST=SU',
   'RRULE:FREQ=WEEKLY;BYDAY=TU;UNTIL=20110429T000000;WKST=SU',
   'RRULE:FREQ=WEEKLY;BYDAY=TU,TH,FR;UNTIL=20110429T000000;WKST=SU',
   'BYDAY=MO,WE;FOO;BYDAY=TU,TH;BAR',
   );
 ;;
 for my $test (@tests) {
   my @bydays = $test =~ m{ (?: BYDAY= | \G ,) ([A-Z]+) }xmsg;
   print qq{@bydays};
   }
"
TU TH
TU
TU TH FR
MO WE TU TH