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Why does '$$' means an empty line in agrep -d '$$'

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# 1  
Old 02-18-2009
Why does '$$' means an empty line in agrep -d '$$'
Hi all,

Why does '$$' means an empty line in agrep -d '$$'. I did not know that '$$' can be seen as an empty line in regular expression. So did I miss something here ?

Thanks!
# 2  
Old 02-18-2009
It looks like a bug. Contact the authors.

PS: By "empty line", you mean the end of the line, right?
Code:
$ grep 's$$' /etc/hosts
$ agrep 's$$' /etc/hosts
# Do not remove the following line, or various programs
$$ agrep --version
agrep (TRE agrep) 0.7.5

Copyright (c) 2002-2006 Ville Laurikari.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

# 3  
Old 02-18-2009
Actually, wait a minute. I just remembered agrep uses the PCRE, and lo-and behold, grep -P returns the same thing! But perl does not have the bug. So either it's in the Perl-compatible regular expression libary, or it's in the parsing.
Code:
$ grep -P 's$$' /etc/hosts
# Do not remove the following line, or various programs
$ perl -n -e 'print if /s$$/;' /etc/hosts
$

# 4  
Old 02-18-2009
my solaris grep version seems to work if you do this:
Code:
grep 'a\$\$' file

# 5  
Old 02-18-2009
By empty line I mean a new line without any characters
Thanks for the reply.

By empty line I mean a new line without any characters (just ~n).
It is not just TRE agrep, for all the agrep versions I find, all their help files come with an example like this,

agrep -d '$$' pattern foo will output all paragraphs (separated by an empty line) that contain pattern.


Either they just copy paste the example or we did miss something here and $$ does mean "empty line". I feel really confused now.
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