You are searching for "something preceeded by".
in reply to Re^2: split on zero-length pattern
in thread split on zero-length pattern
First, that is something *positive*. Why do you want to use a a negative look-around? You are searching for something that is preceeded by, not for something that is *not* preceeded by.
Second, a look-ahead looks *ahead* for the pattern specified (update: maybe better put: looks if the specified pattern is ahead of the look-ahead assertion). For every look-around in perlre there is a short example given. The example for negative look-
behindahead is /foo(?!bar)/, saying "match a foo that is *not* *followed* by "bar". So in your code you said effectively "match anything that is not followed by a dot and a digit".