my login.txt file contains following entries
abc def
abc 123
def abc
abc de
tha ewe
when i do the positive lookahead using perl, i'm getting the following result
cat login.txt | perl -ne 'print if /(?)abc\s(?=def)/'
abc def
when i use grep i'm getting the following result
cat login.txt | grep -P '(?<=abc)\s(?=def)'
abc def
negative lookahed results as follows from perl and grep.
cat login | perl -ne 'print if /(?)abc\s(?!def)/'
abc 123
def abc
abc de
grep result
cat login.txt | grep -P '(?<=abc)\s(?!def)'
abc 123
abc de
perl matched the def abc for the negative lookahead. but it shouldn't matched def abc, as i'm checking abc then def pattern. grep returning the correct result.
is something missing in my perl pattern ?
grep does not include the newline in the string it checks against the regex, so abc\s does not match when abc is at the end of the line. chomp in perl or use the -l command line option and you will see similar results.
I'm not sure why you were making other changes between the perl and grep regexes; what was the (?) supposed to accomplish?
I would try anchoring your regex like so:
/(^abc\s+(?!def).+)/
This would capture:
abc 123
abc de
The (?) at the beginning of your negative lookahead regex is redundant
In your perl -ne 'print if /(?)abc\s(?!def)/' you asking perl to find abc, then space, then string shouldn't be def. This is successfully matches with def abc, because there is no def after abc here and \s matches with newline.
来源：https://stackoverflow.com/questions/20656914/positive-negative-lookahead-with-grep-and-perl