I have a database table containing regular expresions used against strings (sequences of characters to be used in DNA oligo synthesis) as part of a QC process. So far, this has worked great. But now I have a condition that I'm having trouble writing the regex for.

The string is required to contain only alpha characters. Case is not important, and mixed cases are allowed, so the i modifier is used for all match expresions.

So here's the outline of the new condition. Any number of A,T,C and G are always allowed. Some orders may contain symbols representing degenerate possitions however. For example, R may be used to represent a position that can be either A or G. The total list of possible alternate symbols is R,Y,M,K,S,W,H,B,V,D, and N. Although any of them may be used, only a total of two different alternate codes can be used in a given string (mechanical limitation of the synthesis machine).

So, the chalange is to have a single regular expresion that will match a sequence containing any number of A,T,C,G and any number of no more than two different characters from the above alternate codes.

Thanks in advance for whatever wisdom and guidence you can bestow!