In ini_aug_regex_prepare(), regcomp(7) is called without passing REG_EXTENDED. This results in POSIX Basic regular expressions being used. This is undesirable largely because it is unexpected and unfamiliar to most people who have worked with regular expressions before. Basic REs lack many of the features of the extended set; as per the man page, "'|', '+', and '?' are ordinary characters and there is no equivalent for their functionality.".

All that said, I can understand a case for not wanting extended regexes (if I understand correctly, extended supports backreferences while basic does not). However, either way the decision is made, it does need to be specified in the documentation; having to read the implementation to figure this out is a bit much.