What's the use of "?!" in this RewriteCond?

Question

I frequently see the ?! combination in RewriteCond given in .htaccess files, like this:

RewriteCond %{REQUEST_URI} ^(?!/slim_demo/index\.php).*$

To my understanding, this directive checks if /slim_demo/index.php isn't part of the REQUEST_URI. Which is odd; what is the use of the ? at the very front, especially when it has no preceding character to match. The documentation says that ? "makes the match optional", but the only thing preceding the ? is the grouping bracket. Does it mean that ? is making grouping optional? Makes so sense!

What is the mystery?

It's a [negative lookahead](http://www.regular-expressions.info/lookaround.html) — arco444, Jul 14 '15 at 11:50
@arco444 Ah, I wish I had known how to search about it! Thanks a lot. — ankush981, Jul 14 '15 at 11:58

score 1 · Accepted Answer · answered Jul 14 '15 at 11:55

Negative Lookahead After the Match: \d+(?!\d| dollars) Sample Match: 100 in 100 pesos Explanation: \d+ matches 100, then the negative lookahead (?! dollars) asserts that at that position in the string, what immediately follows is neither a digit nor the characters " dollars"

What's the use of "?!" in this RewriteCond?

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