YAPE::Regex::Explain to the rescue.

The outer "(?x-ims:" is what qr makes from the /x flag, and the question mark in the ")? end of grouping" is reflected in the "optional" at the comment at the start ot said group

perl -wMstrict -MYAPE::Regex::Explain -e "print YAPE::Regex::Explain->
+new(qr/( [a-zA-Z]+ (?: ' [a-zA-Z]{1,2} )? )/x)->explain;"
The regular expression:

(?x-ims:( [a-zA-Z]+ (?: ' [a-zA-Z]{1,2} )? ))

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?x-ims:                 group, but do not capture (disregarding
                         whitespace and comments) (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n):
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    [a-zA-Z]+                any character of: 'a' to 'z', 'A' to 'Z'
                             (1 or more times (matching the most
                             amount possible))
----------------------------------------------------------------------
    (?:                      group, but do not capture (optional
                             (matching the most amount possible)):
----------------------------------------------------------------------
      '                        '\''
----------------------------------------------------------------------
      [a-zA-Z]{1,2}            any character of: 'a' to 'z', 'A' to
                               'Z' (between 1 and 2 times (matching
                               the most amount possible))
----------------------------------------------------------------------
    )?                       end of grouping
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
[download]