troubleshooting Question

Regular Expression - Help!!

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A4eIT asked on
Visual Basic.NET
7 Comments1 Solution223 ViewsLast Modified:
hi

i need to write a regular expression that will do the following:

allow input of any combination of ABCD without duplicates,
 
ie A, B, C, D, AB, AC, AD, ABC, ABD, ABCD, DBA, DAB, DA etc, but NOT AA, BAB, ABCC, (any duplicated letters)

please help, 500 points!

many thanks
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