Positive/Negative lookahead with grep and perl

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.

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?