Your have to make sure no position starts matches /ab/.

/^a(?:(?!ab).)*b$/
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(?:(?!regexp).)
is to regexps as
[^chars]
is to chars.

Update: Not quite. /$pre(?(?!$regexp).)*$post/ only works when /$regexp/ can't match something /.$post/ matches.

It doesn't work in this case because /ab/ can match something that /.b/ matches. It'll fail if the input is a000ab, for example.

However, I suspect you have something closer to the following, which works fine:

/
   <tag>             # Start tag
   (?:(?!</tag>).)*  # Body
   </tag>            # End tag
/x
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