shell> perl -Mre=debug -e '$_="1/2 MILE"; m{ 1/2 \s* (?!MILE) }x'
Freeing REx: `,'
Compiling REx ` 1/2 \s* (?!MILE) '
size 11 first at 1
   1: EXACT <1/2>(3)
   3: STAR(5)
   4:   SPACE(0)
   5: UNLESSM[-0](11)
   7:   EXACT <MILE>(9)
   9:   SUCCEED(0)
  10:   TAIL(11)
  11: END(0)
anchored `1/2' at 0 (checking anchored) minlen 3 
Guessing start of match, REx ` 1/2 \s* (?!MILE) ' against `1/2 MILE'...
Found anchored substr `1/2' at offset 0...
Guessed: match at offset 0
Matching REx ` 1/2 \s* (?!MILE) ' against `1/2 MILE'
  Setting an EVAL scope, savestack=3
   0 <> <1/2 MILE>        |  1:  EXACT <1/2>
   3 <1/2> < MILE>        |  3:  STAR
                           SPACE can match 1 times out of 32767...
  Setting an EVAL scope, savestack=3
   4 <1/2 > <MILE>        |  5:    UNLESSM[-0]
   4 <1/2 > <MILE>        |  7:      EXACT <MILE>
   8 <1/2 MILE> <>        |  9:      SUCCEED
                                could match...
                              failed...
   3 <1/2> < MILE>        |  5:    UNLESSM[-0]
   3 <1/2> < MILE>        |  7:      EXACT <MILE>
                                failed...
   3 <1/2> < MILE>        | 11:    END
Match successful!
Freeing REx: ` 1/2 \s* (?!MILE) '