The string patterns article is missing some information.
Frontier pattern %f[set]
Although undocumented in 5.1 manual, it does exist.
According to the 5.3 manual,
a frontier pattern; such item matches an empty string at any position such that the next character belongs to set and the previous character does not belong to set. The set set is interpreted as previously described. The beginning and the end of the subject are handled as if they were the character \0.
Anchors ^ and $
^ at the beginning anchors the match to the beginning of the string, and $ at the end anchors the match to the end of the string.
Empty capture ()
Captures the current string position (a number)
Nested capture
Little detail is given about captures, but I think it is worth mentioning that captures are ordered by their left parenthesis.
%n, for n between 1 and 9; such item matches a substring equal to the n-th captured string
The devhub also states that the %bxy pattern item is a character class, which it is not.