Regular expression negative lookahead

Question

In my home directory I have a folder drupal-6 14 that contains the Drupal platform   From this directory I use the following command   find drupal-6 14 -type f -iname       grep -P  drupal-6 14    sites    all  default        xargs tar -czf drupal-6 14 tar gz   What this command does is gzips the folder drupal-6 14  excluding all subfolders of drupal-6 14 sites  except sites all and sites default  which it includes   My question is on the regular expression   grep -P  drupal-6 14    sites    all  default        The expression works to exclude all the folders I want excluded  but I don t quite understand why   It is a common task using regular expressions to     Match all strings  except those that don t contain subpattern x  Or in other words  negating a subpattern    I  think  I understand that the general strategy to solve these problems is the use of negative lookaheads  but I ve never understood to a satisfactory level how positive and negative look ahead behind s work   Over the years  I ve read many websites on them  The PHP and Python regex manuals  other pages like http   www regular-expressions info lookaround html and so forth  but I ve never really had a solid understanding of them   Could someone explain  how this is working  and perhaps provide some similar examples that would do similar things   -- Update One   Regarding Andomar s response  can a double negative lookahead be more succinctly expressed as a single positive lookahead statement   i e Is     drupal-6 14    sites    all  default        equivalent to    drupal-6 14    sites    all  default             -- Update Two   As per  andomar and  alan moore - you can t interchange double negative lookahead for positive lookahead

User · Answer

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.

User · Answer

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 a 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

User · Answer

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

[regex] Regular expression negative lookahead

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