The regular expression:
(?x-ims:Hello\#\ World\\E)
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):
----------------------------------------------------------------------
  Hello                    'Hello'
----------------------------------------------------------------------
  \#                       '#'
----------------------------------------------------------------------
  \                        ' '
----------------------------------------------------------------------
  World                    'World'
----------------------------------------------------------------------
  \\                       '\'
----------------------------------------------------------------------
  E                        'E'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
********************
The regular expression:
(?x-ims:Hello\#\ World)
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):
----------------------------------------------------------------------
  Hello                    'Hello'
----------------------------------------------------------------------
  \#                       '#'
----------------------------------------------------------------------
  \                        ' '
----------------------------------------------------------------------
  World                    'World'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------