You have to remember that the regex engine tries as hard as it can to make the match work.

"4 1/2 MILE RD" =~ m!(1/2)\s*(?!MILE)!;
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First, the engine slurps up all the space after the "1/2" with the \s* and finds that it doesn't match because of your assertion. Next, it backtracks by slurping all but one space with the \s*. At that point, your assertion "MILE" doesn't match " MILE" (with a leading space) so it is successful.

-sauoq
"My two cents aren't worth a dime.";