I agree that a generic parser as suggested by ikegami is probably the best approach.

However, here's a regex solution to the specific problem. Note this assumes only one BYDAY field per line/record; if there are more, all field values are extracted indiscriminately.

>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
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