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

perl -ne 'print if /(?)abc\s(?!def)/'

To begin, as fugi stated, the (?) is an empty non-capturing group, and matches anything, so it does nothing.

Therefore as written, this regex matches the literal string abc followed by a single [:space:OR:tab:OR:newline], not followed by the literal string def.

Because \s matches a newline character and you did not chomp the trailing newline characters as you processed each line, def abc matches because (?)abc\s in the regex matches abc[:newline:] which is followed by $ (the end-of-line anchor, not def).

The corrected regex (accounting for the redundant (?)) would be:

perl -ne 'print if /(?<=abc)\s(?!def)/'

...which matches a single [:space:OR:tab:OR:newline] which is preceded by abc and not followed by def.

This still will match def abc, because once again, \s matches the [:newline:], which is preceded by abc and followed by $ (the end-of-line anchor, not def).

Either chomp the [:newline:] before evaluating the regex in Perl, or use the character class [ \t] (if you need to account for tab characters) instead of \s:

perl -ne 'print if /(?<=abc)[ \t](?!def)/'

Or simply

perl -ne 'print if /(?<=abc) (?!def)/'