C:\>perl -MYAPE::Regex::Explain -e "print YAPE::Regex::Explain->new(qr/^Posted by (.*?) on /)->explain()"
The regular expression:

(?-imsx:^Posted by (.*?) on )

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  Posted by                'Posted by '
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    .*?                      any character except \n (0 or more times
                             (matching the least amount possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
   on                      ' on '
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------