Why? Because negative lookbehind is satisfied at any position except immediately after the given subpattern. Thus your (.*?) is free to match at the very beginning. Using a lookahead may accomplish what you had in mind (if $1 must contain the start tag). 

   if (/^(?!TAGS)(.*?)TAG2/x ) {
      ...

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In reply to Re: negative look behind (again) by Anonymous Monk
in thread negative look behind (again) by cmic

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