如何在n次构造中强制匹配另一个字符串重复n次

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 the following pattern that is not working as i want it to:

^(fr|sa|su|mo|tu|we|th)?(?(1)(,fr|,sa|,su|,mo|,tu|,we|,th)){0,5}$

This pattern successfully matches the desired input like the following:

fr

mo

fr,sa

fr,tu

su,mo,tu,we,fr,sa

but it also matches the following wrong enteries:

fr,fr,fr,fr

sa,sa,sa,sa

I want a way to force the second group to repeat only different values included in the OR construct. is there a way regex can do that?

Answer 1

You may use a (?!.*([az]{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([az]{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 .