Positive/Negative lookahead with grep and perl

Question

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 t

Answer 1

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