regex to match a word with unique (non-repeating) characters

Solution 1:

Try this, it might work,

^(?:([A-Za-z])(?!.*\1))*$

Explanation

Assert position at the beginning of a line (at beginning of the string or after a line break character) «^»
Match the regular expression below «(?:([A-Z])(?!.*\1))*»
   Between zero and unlimited times, as many times as possible, giving back as needed (greedy) «*»
   Match the regular expression below and capture its match into backreference number 1 «([A-Z])»
      Match a single character in the range between “A” and “Z” «[A-Z]»
   Assert that it is impossible to match the regex below starting at this position (negative lookahead) «(?!.*\1)»
      Match any single character that is not a line break character «.*»
         Between zero and unlimited times, as many times as possible, giving back as needed (greedy) «*»
      Match the same text as most recently matched by capturing group number 1 «\1»
Assert position at the end of a line (at the end of the string or before a line break character) «$»

Solution 2:

You can check whether there are 2 instances of the character in the string:

^.*(.).*\1.*$

(I just simply capture one of the character and check whether it has a copy elsewhere with back reference. The rest of .* are don't-cares).

If the regex above match, then the string has repeating character. If the regex above doesn't match, then all the characters are unique.

The good thing about the regex above is when the regex engine doesn't support look around.

Apparently John Woo's solution is a beautiful way to check for the uniqueness directly. It assert at every character that the string ahead will not contain the current character.