$ perl -Mre=debug -le " $_ = q{this TON that}; m{this \s+ (?!NOT)\w+ \s+that}msgx; "
Compiling REx "this \s+ (?!NOT)\w+ \s+that"
Final program:
   1: EXACT <this> (3)
   3: PLUS (5)
   4:   SPACE (0)
   5: UNLESSM[0] (11)
   7:   EXACT <NOT> (9)
   9:   SUCCEED (0)
  10: TAIL (11)
  11: PLUS (13)
  12:   ALNUM (0)
  13: PLUS (15)
  14:   SPACE (0)
  15: EXACT <that> (17)
  17: END (0)
anchored "this" at 0 floating "that" at 7..2147483647 (checking floating) minlen 11
Guessing start of match in sv for REx "this \s+ (?!NOT)\w+ \s+that" against "this TON that"
Found floating substr "that" at offset 9...
Found anchored substr "this" at offset 0...
Guessed: match at offset 0
Matching REx "this \s+ (?!NOT)\w+ \s+that" against "this TON that"
   0 <> <this TON t>         |  1:EXACT <this>(3)
   4 <this> < TON that>      |  3:PLUS(5)
                                  SPACE can match 1 times out of 2147483647...
   5 <this > <TON that>      |  5:  UNLESSM[0](11)
   5 <this > <TON that>      |  7:    EXACT <NOT>(9)
                                      failed...
   5 <this > <TON that>      | 11:  PLUS(13)
                                    ALNUM can match 3 times out of 2147483647...
   8 <his TON> < that>       | 13:    PLUS(15)
                                      SPACE can match 1 times out of 2147483647...
   9 <his TON > <that>       | 15:      EXACT <that>(17)
  13 <his TON that> <>       | 17:      END(0)
Match successful!
Freeing REx: "this \s+ (?!NOT)\w+ \s+that"