Add Explanation of RegEx and Reason for AASd-130

[g1zzm0]

This was necessary because there was a broad disagreement in different committees and persons about what the AASd-130 constraint says, what the RegEx in AASd-130 means and what the reason for the definition of the constraint was.

[BirgitBoss]

is it V3.0.1 or V3.1: what is the related issue?

[JoergNeidig]

@sebbader-sap @sebbader Can you please review this issue?

[sebbader-sap]

The problem is that we can "serialise" AASd-130 into different regex patterns due to the fact that regex itself is underspecified. I tried to explain our decision for the UTF-16 representation a bit better, and that this representation might change when UTF-32-enabled regex libraries become more common.

[BirgitBoss]

@g1zzm0 can you please have a look at the suggested changes of Sebastian and the merge?