Vim lookahead / lookbehind reference

This is another simple Vim lookahead / lookbehind reference.

\@< Lookbehind \@< \@ Lookahead \@  
\(pattern1\)\@<=pattern2
pattern2\(pattern3\)\@=
= Positive =
\(pattern1\)\@<!pattern2
pattern2\(pattern3\)\@!
! Negative !

Lookbehind - search for pattern2 proceeded by pattern1.

Lookahead - search for pattern2 followed by pattern3.

\(pattern1\)\@<=pattern2\(pattern3\)\@=

Search for pattern1 (not) proceeded or followed by pattern2.

Note: The enclosing parenthesis \(...\) are optional when pattern1 or pattern3 are one character.