vim - replace command conditioned by preceding and following atoms

Going from:

John has one hundred apples
Jane has twelve hundred and fifty seven apples
Mary has thirty seven apples

to

John has one_hundred apples
Jane has twelve_hundred_and_fifty_seven apples
Mary has thirty_seven apples

in vim:

.,$s/\(has .*\)\@<= \(.* apples\)\@=/_/g