This looks like overkill.

If a string is required to contain all of an "a", a "b" and a "c" and its length is limited to 3, then it can only contain one of each.

So this ought to suffice: 

/^(?=.*a)(?=.*b)(?=.*c).{3}\z/

[download]

Of course, yours is the way to go if the length is not limited to 3.

In reply to Re^2: REgular expression to check the string that allows "a","b" and "c" to occur only once in any order. by bart
in thread REgular expression to check the string that allows "a","b" and "c" to occur only once in any order. by isha

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