TElPGPRegularExpressionSignatureSubpacket is a descendant of TElPGPSignatureSubpacket class.
The following paragraph is taken from RFC 2440 - OpenPGP messages format, part 18.104.22.168.:
«Used in conjunction with trust signature packets (of level > 0) to limit the scope of trust that is extended. Only signatures by the target key on user IDs that match the regular expression in the body of this packet have trust extended by the trust signature subpacket. The regular expression uses the same syntax as the Henry Spencer's "almost public domain" regular expression package. »