Separating elements from a list according to preceding element

[Rob Cowie]

I'm having a bit of trouble with this so any help would be gratefully
recieved...

After splitting up a url I have a string of the form
'tag1+tag2+tag3-tag4', or '-tag1-tag2' etc. The first tag will only be
preceeded by an operator if it is a '-', if it is preceded by nothing,
'+' is to be assumed.

Using re.split, I can generate a list that looks thus:
['tag1', '+', 'tag2', '+', 'tag3', '-', 'tag4']

I wish to derive two lists - each containing either tags to be
included, or tags to be excluded. My idea was to take an element,
examine what element precedes it and accordingly, insert it into the
relevant list. However, I have not been successful.

Is there a better way that I have not considered? If this method is
suitable, how might I implement it?

Thanks all,

Rob Cowie