This is due to the nature of * and while it is a greedy quantifier, it is also quite a lazy quantifier. In this case \s* doesn't match any space, since it doesn't have to, and subsequently " MILE" is matched which doesn't match "MILE" so it's a successful match. Hopefully this quick oneliner and it's output should make that explanation clearer 
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) '

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HTH

_________
broquaint

In reply to Re: regex negative lookahead behaviour by broquaint
in thread regex negative lookahead behaviour by shemp

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