Question

I have to validate a string which can contain numbers from 1 to 7, and maximum length allowed is 7.

([1-7]){0,7}

Condition: No numbers should be repeated in the string.

eg:

12345  true;
11345  false  (1 is repeated) ;
98014  false  (0,8,9 are invalid); 

Answer 1

This can be done in a single regex:

^(?!.*(.).*\1)[1-7]{0,7}$

The lookahead assertion checks that all characters in the string are unique, and the actual regex only allows 0-7 digits between 1 and 7.

In Java:

boolean foundMatch = subjectString.matches("^(?!.*(.).*\\1)[1-7]{0,7}$");

Of course you can make the lookahead fail faster by replacing each . with [1-7], but for clarity’s sake I’ve chosen not to. (And you can drop the ^ and $ anchors if you use the .matches() method since they are implicit in that case).