Could someone help me about to create a posix regex?

1
votes

could someone help me to create a posix regex which checks if the entered string follows the following pattern:

Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday

Or

Monday, Wednesday, Friday

Or

Sunday

Then:

  1. Days of the week followed by ", "
  2. I can't repeat a day of the week
  3. The last day of a string must not have ", "
1
javaregexpostgresqlposix-ere
Yes, if you try something and have a problem which you demonstrate here with code and describe in detail, then you will surely find help here. For easy tinkering try regex101.com Generally please take the tour and read How to Ask. – Yunnosch
I just don't know how to do it, so how could I explain it?? – user13998900

1 Answers

0
votes

If the order of days doesn't matter, use:

^((Monday|Tuesday|Wednesday|Thursday|Friday|Saturday|Sunday)(, (?!$)(?!.*\2)|$))*$

See demo here.

Explanation:

  • The inner parenthesis are used for expressing the disjunction of days.
  • ^ matches the begin of the string.
  • $ matches the end of the string.
  • (?!$) means that comma must not be follwed by the end of string.
  • (?!.*\2) means not followed by any number of characters (.*) and the second group (\2), that is the innermost one.

About the last point, I suggest you to google about regex matching groups, lookarounds backreferences