import re
s="""line1
line2
field: [orders.cancelled,orders.delivered,orders.reached
orders.pickup,orders.time]
some line
some line"""
d = {'cancelled':'cancelled_at', 'deliver':'xxx'}
rx = fr"(?<=\.)(?:{'|'.join(d)})\b"
print(rx)
print( re.sub(rx, lambda x: d[x.group()], s) )