How to force matching a different string in an OR construct repeated n times

Question

I am new to regex, i am trying to validate a field the user is entering weekend days in a comma separated format and using 2 characters abbreviation for the day. i developed

Answer 1

You may use a (?!.*([a-z]{2}).*\1) negative lookahead at the start of regex to disallow repeating 2-letter values in the string (due to the string format, you do not even need word boundaries or comma context checks in the lookahead):

^(?!.*\b([a-z]{2})\b.*\b\1\b)(fr|sa|su|mo|tu|we|th)?(?:,(?:fr|sa|su|mo|tu|we|th)){0,5}$

See the regex demo.

The (?!.*\b([a-z]{2})\b.*\b\1\b) is a negative lookahead that fails the match if there is a duplicate two-letter chunk in the string.

Java demo:

String day = "fr|sa|su|mo|tu|we|th";
String pattern = "(?!.*\\b([a-z]{2})\\b.*\\b\\1\\b)(?:" + day + ")?(?:,(?:" + day + ")){0,5}";
if (s.matches(pattern)) {
    System.out.println("Valid!");
} else {
    System.out.println("Invalid!");
}

Note that String#matches requires a full string match, so ^ and $ are not required.

Note you may shorten the day part using character classes, fr|sa|su|mo|tu|we|th => fr|s[au]|mo|t[uh]|we.