(Positive Negative) Look(ahead behind) 
echo "mona lisa" | ag "a"
mona lisa
   -    -

echo "mona lisa" | ag "a(?= )"
mona lisa
   -

echo "mona lisa" | ag "a(?! )"
mona lisa
        -

echo "mona lisa" | ag "(?<=n)a"
mona lisa
   -

echo "mona lisa" | ag "(?<!n)a"
mona lisa
        -

Positive

=

Negative

!

Lookahead

(? )

Lookbehind

(?< )

Positive Lookahead

(?= )

Positive Lookbehind

(?<= )

Negative Lookahead

(?! )

Negative Lookbehind

(?<! )