Work in progress (D10315) is going to make egrep_empty_invalid an
actually invalid regex, to be consistent with the equivalent BRE "{"
behavior, when using regex(3).
Any non-0 exit value is acceptable, depending on how the installed grep
interprets the expression. GNU grep interprets it as non-matching, and
in the future BSD grep will interpret it is an error.
Submitted by: Kyle Evans <kevans91 at ksu.edu>
Reviewed by: cem, ngie
Differential Revision: https://reviews.freebsd.org/D10572`
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