[Boost-users] Boost Regex - Alternation documentation example

According to the Boost Regex documentation for Alternation (http://www.boost.org/doc/libs/1_64_0/libs/regex/doc/html/boost_regex/syntax/...) Quoting a piece of the documentation: ----- |abc is not a valid expression, but (?:)|abc is and is equivalent, also the expression: (?:abc)?? has exactly the same effect. ----- Is the last expression: (?:abc)?? correct? Why does it need two '?' operators instead of only one? If I try this expression with the input "abc" the Boost engine does not match "abc" as I'd have expected. However the expression: (?:abc)? matches as expected. Nick