Negative Lookbehind in vim

very similar to the last post, but this time for _not_ matching text that is preceded with some other text, and this time using the vim regex engine which is syntactically a little different
/\(Start\)\@<!Date
This will match the 'Date' in 'EndDate' and 'YesterdaysDate' but will not match 'StartDate'

and what the hell, while I'm lookup up vim syntax, here's negative lookahead:
/Start\(Date\)\@!
will match the 'Start' in 'Starting but not in 'StartDate'

Positive lookahead:
/Start\(Date\)\@=
will match the 'Start' in 'StartDate' but not in 'Starting

Positive lookbehind
/\(Start\)\@<=Date
will match the 'Date' in 'StartDate' but not in 'EndDate' and 'YesterdaysDate'

Regular Expressions - Negative Lookahead

/a(?!b)/ will match an a that's not followed by a b (with the not-b character not being included in the match)

this can be built upon to match ranges that dont contain a string.

/^((?!DONT_MATCH).)*$/ will match full lines that dont contain the string "DONT_MATCH"


An example using negative look ahead in order to close <p> tags in poorly formed html:
$ irb
>> x="<p>an un-closed paragraph tag <p>and a properly formed paragraph</p>"
=> "<p>an un-closed paragraph tag <p>and a properly formed paragraph</p>"
>> x.sub(/<p>((?!</p><p>).)*/,"")
=> "<p>and a properly formed paragraph</p>"
>> x.sub(/<p>(((?!</p><p>).)*)/,"<p>\\1</p>")
=> "<p>an un-closed paragraph tag </p><p>and a properly formed paragraph</p>"


!@#$%^!! blogger seems to have "helpfully" correcting my bad html example when I tried to use the wysiwyg editor. attempt 2 as plain html.