A quick note about these two assertions, let's start with positive lookahead:



"Assert that the regex below can be matched, starting at this position".



Let's go with this little example:



Regex:

\s+\w+(?=\.)



This would mean, match a space between one and many times followed by a word character between one an many times only if there is a dot following the previous match:



Eg.

andy mypass ok orange..see what is going on



The matched text here would be orange (Because there was a dot after "orange", ah yes, and many spaces before).



Now, let's go with the other case, negative lookahead:

"Assert that it is impossible to match the regex below starting at this position".

The regex changes to

\s+\w+(?!\.)



Do you see the difference?. this should read now:



Match a space between one and many times followed by a word character between one and many times only if the following character is not a dot, so what is matched now using our previous example:





mypass (Because after "mypass" was a space)

ok (Same reason using the word "ok")

orang (Because after "orang" there is an e)

see (Because after "see" there is a space)

.... and so on