Regular expression negative lookahead

Solution 1:

A negative lookahead says, at this position, the following regex can not match.

Let's take a simplified example:

a(?!b(?!c))

a      Match: (?!b) succeeds
ac     Match: (?!b) succeeds
ab     No match: (?!b(?!c)) fails
abe    No match: (?!b(?!c)) fails
abc    Match: (?!b(?!c)) succeeds

The last example is a double negation: it allows b followed by c. The nested negative lookahead becomes a positive lookahead: the c should be present.

In each example, only the a is matched. The lookahead is only a condition, and does not add to the matched text.

Solution 2:

Lookarounds can be nested.

So this regex matches "drupal-6.14/" that is not followed by "sites" that is not followed by "/all" or "/default".

Confusing? Using different words, we can say it matches "drupal-6.14/" that is not followed by "sites" unless that is further followed by "/all" or "/default"

Solution 3:

If you revise your regular expression like this:

drupal-6.14/(?=sites(?!/all|/default)).*
             ^^

...then it will match all inputs that contain drupal-6.14/ followed by sites followed by anything other than /all or /default. For example:

drupal-6.14/sites/foo
drupal-6.14/sites/bar
drupal-6.14/sitesfoo42
drupal-6.14/sitesall

Changing ?= to ?! to match your original regex simply negates those matches:

drupal-6.14/(?!sites(?!/all|/default)).*
             ^^

So, this simply means that drupal-6.14/ now cannot be followed by sites followed by anything other than /all or /default. So now, these inputs will satisfy the regex:

drupal-6.14/sites/all
drupal-6.14/sites/default
drupal-6.14/sites/all42

But, what may not be obvious from some of the other answers (and possibly your question) is that your regex will also permit other inputs where drupal-6.14/ is followed by anything other than sites as well. For example:

drupal-6.14/foo
drupal-6.14/xsites

Conclusion: So, your regex basically says to include all subdirectories of drupal-6.14 except those subdirectories of sites whose name begins with anything other than all or default.