Using regex lookahead, egrep

问题

If your file contains

apples are good
apple cider is also good

Why would egrep '(?=apples)app' file fail to pick up any lines?

Using egrep 2.5.1 on MAC

回答1:

Extended regular expression doesn't have positive look-ahead feature. See the regex flavor comparison

来源：https://stackoverflow.com/questions/10645542/using-regex-lookahead-egrep