The regular expression:

(?i-msx:\bold\b)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?i-msx:                 group, but do not capture (case-insensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  \b                       the boundary between a word char (\w) and
                           something that is not a word char
----------------------------------------------------------------------
  old                      'old'
----------------------------------------------------------------------
  \b                       the boundary between a word char (\w) and
                           something that is not a word char
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------