what should egrep '|root' /etc/passwd print?

[Richard A. O'Keefe]

In article <8202 at alice.UUCP> andrew at 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?)