If you want to block spam on IPv6, then you can start by rejecting connections to SMTP from any IPv6 that do not have a PTR. No need to analyze the format of the PTR.
It is in several recommendations that a sending email IP must have a PTR. That ISPs will not do a PTR on all IPv6 but only on static IPv6, improves the spam blocking feature above. No need to maintain list of dynamic IP space...
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