Path: utzoo!attcan!uunet!seismo!sundc!pitstop!sun!quintus!ok
From: ok@quintus.uucp (Richard A. O'Keefe)
Newsgroups: comp.unix.wizards
Subject: Re: what should egrep '|root' /etc/passwd print?
Message-ID: <410@quintus.UUCP>
Date: 16 Sep 88 00:35:03 GMT
References: <44414@beno.seismo.CSS.GOV> <68203@sun.uucp> <8202@alice.UUCP>
Sender: news@quintus.UUCP
Reply-To: ok@quintus.UUCP (Richard A. O'Keefe)
Organization: Quintus Computer Systems, Inc.
Lines: 30

In article <8202@alice.UUCP> andrew@alice.UUCP (Andrew Hume) writes:
>it is true that youhave to think carefully about null regular expressions.
Right.
...
>i think that it is clear that grep '' file is always a syntax error;
>there is no useful interpretation of such a regular expression.
Wrong.

(1) The empty r.e. is the mathematically obvious way of writing a
    pattern which matches the empty string.

(2) grep -c '' file		- print the number of lines in the file
    grep -n '' file		- print lines with line numbers
    ls | egrep '\.c(|\.BAK)$'	- find *.c and *.c.BAK files

    The first two of these work fine in SunOS 3.2, the last one is the
    one I would really like, and it gets a syntax error.  There are
    several things I could do to make it acceptable, but why should I?
    It's clear as it stands, and csh would let me use *.c{,.BAK} as a
    pattern.
    
    It is true that the System V version of grep is broken here: if
    you try "grep -c '' file" you are told
	grep: RE error 41: No remembered search string
    which is absurd.  In order to write a pattern which will match the
    empty string, you are obliged to write something like
	grep -c 'x\{0\}' file
    It's crazy to allow a complicated hack like this but not the
    direct expression of the idea.  (Why is the \{..\} feature in
    grep, but not in egrep?)