There is a module YAPE::Regex::Explain which can help you to understand regular expressions. Here the output for your expression:

The regular expression:

(?-imsx:(?:(?<=\.|\!|\?)(?<!Mr\.|Dr\.)(?<!U\.S\.A\.)\s+(?=[A-Z])))

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):
----------------------------------------------------------------------
  (?:                      group, but do not capture:
----------------------------------------------------------------------
    (?<=                     look behind to see if there is:
----------------------------------------------------------------------
      \.                       '.'
----------------------------------------------------------------------
     |                        OR
----------------------------------------------------------------------
      \!                       '!'
----------------------------------------------------------------------
     |                        OR
----------------------------------------------------------------------
      \?                       '?'
----------------------------------------------------------------------
    )                        end of look-behind
----------------------------------------------------------------------
    (?<!                     look behind to see if there is not:
----------------------------------------------------------------------
      Mr                       'Mr'
----------------------------------------------------------------------
      \.                       '.'
----------------------------------------------------------------------
     |                        OR
----------------------------------------------------------------------
      Dr                       'Dr'
----------------------------------------------------------------------
      \.                       '.'
----------------------------------------------------------------------
    )                        end of look-behind
----------------------------------------------------------------------
    (?<!                     look behind to see if there is not:
----------------------------------------------------------------------
      U                        'U'
----------------------------------------------------------------------
      \.                       '.'
----------------------------------------------------------------------
      S                        'S'
----------------------------------------------------------------------
      \.                       '.'
----------------------------------------------------------------------
      A                        'A'
----------------------------------------------------------------------
      \.                       '.'
----------------------------------------------------------------------
    )                        end of look-behind
----------------------------------------------------------------------
    \s+                      whitespace (\n, \r, \t, \f, and " ") (1
                             or more times (matching the most amount
                             possible))
----------------------------------------------------------------------
    (?=                      look ahead to see if there is:
----------------------------------------------------------------------
      [A-Z]                    any character of: 'A' to 'Z'
----------------------------------------------------------------------
    )                        end of look-ahead
----------------------------------------------------------------------
  )                        end of grouping
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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