i think this is the correct regexp for the job:

^(?!i|ddi)[a-z_]+(\d+(?:\.\d+)?)(?:[a-z](\d+(?:\.\d+)?))?
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$1 will contain the first dimension, and $2 will contain either the second dimension, or undef if it is missing.

however, it may be better to check for ^i|ddi separately, as others have done.