RegEx: Allow value only once

Question

I have a string of numbers and each number can appear either zero or one time in the string. Can I validate this via RegEx? I could do something like check the character array for duplicates, but would prefer to stick to my normal validation routine.

The following should return "Match"

String thisItemText = "12679";
if(!thisItemText.matches("[1245679]*")) {
    System.out.println("No Match");
} else {
    System.out.println("Match");
}

The following should return "No Match" (note the double "2")

String thisItemText = "122679";
if(!thisItemText.matches("[1245679]*")) {
    System.out.println("No Match");
} else {
    System.out.println("Match");
}

Answer 1

The regex (\d).*\1 will match if there's a repeated digit.

Answer 2

Try this pattern:

"(?<=([0-9]))(?:(?!\\1).)*\\1(?!\\1)"

will match a string that has any repeating digits. So if it does not match then it does not have any repeating digits.