Proposed resolution:

In [re.regiter.cnstr]/2 change the above quoted part by

Effects: Initializes begin and end to point to the beginning and the end of the target sequence designated by the iterator range [a, b), sets pregex to &re, sets flags to f m, then calls regex_search(begin, end, match, *pregex, flags). If this call returns false the constructor sets *this to the end-of-sequence iterator.

In [re.regiter.cnstr]/2 the effects paragraph starts with:

Effects: Initializes begin and end to point to the beginning and the end of the target sequence, sets pregex to &re, sets flags to f,[..]

There are two issues with this description:

1. The meaning of very first part of this quote is unclear, because there is no target sequence provided, instead there are given two parameters a and b, both of type BidirectionalIterator. The mentioned part does not explain what a and b represent.
2. There does not exist any parameter f, but instead a parameter m in the constructor declaration, so this is actually an editorial fix.