Extended pattern matching operators and the =~ operator

[M. Nejat AYDIN]

An example given in the Bash Reference Manual, section 3.2.4.2
Conditional Constructs, seems to imply that extended pattern matching
operators can be used in the string to the right of the '=~' operator,
which is suspicious to me. Quoting from the manual:

  For example, the following will match a line (stored in the shell
  variable line) if there is a sequence of characters in the value
  consisting of any number, including zero, of space characters, zero or
  one instances of ‘a’, then a ‘b’:
    [[ $line =~ [[:space:]]*?(a)b ]]
  That means values like ‘aab’ and ‘ aaaaaab’ will match, as will a line
  containing a ‘b’ anywhere in its value.

Please notice the use of "?(a)" extended pattern matching expression.
Is it an error in the manual, or did I overlook something?