Lookahead and Lookbehind

Look everywhere

Input

Example for reference:

:%s/\(some\)\@<=thing/one/g
searches for all strings starting with some, then matching thing
changes thing into one
end result: something becomes someone

:%s/\(some\)\@

Output

Example for reference:

:%s/\(some\)\@<=thing/one/g
searches for all strings starting with some, then matching thing
changes thing into one
end result: someone becomes someone

:%s/\(some\)\@