How can I transform this Backus–Naur Form expression into a Regex (.Net)?

Question

The expression is:

N | ( 1 { A | B | C | D | E1 | E2 | E3 } )

Meaning the descriptor "N" or one or more of the listed descriptors without repetition.

The best I have got is:

@"^(N|(A|B|C|D|E1|E2|E3){1,})$"

But that does not prevent repetition.

@"^(N|(A{0,1}B{0,1}...)$" 

That prevents repetition but then requires a specific order to the elements, which isn't really OK either.

Any ideas?

(I am not actually sure that the bnf expression itself disallows repetition, but that is what I need.)

Answer 1

Well, you can, but it ain't pretty:

Regex regexObj = new Regex(
    @"^           # Start of string
    (?:           # Either match...
     N            # N
    |             # or...
     (?:          # Match one of the following:
      A(?!.*A)    # A unless followed somewhere later by another A
     |            # or
      B(?!.*B)    # B unless...
     |            # etc. etc.
      C(?!.*C)
     |
      D(?!.*D)
     |
      E1(?!.*E1)
     |
      E2(?!.*E2)
     |
      E3(?!.*E3)
     )+           # one or more times
    )             # End of alternation
    $             # End of string", 
    RegexOptions.IgnorePatternWhitespace);

This solution uses negative lookahead assertions.