We can set RE_NO_SUB for calling regex only to check syntax.  It brings
performance gains in cases to have a lot of enormous epsilon nodes.


$ printf '(%020000d)\n' | sed 's/0/|/g' >pat

(before)
$ time -p env LC_ALL=C src/grep -Ef pat /dev/null
real 6.15
user 4.62
sys 1.52

(after)
$ time -p env LC_ALL=C src/grep -Ef pat /dev/null
real 0.66
user 0.19
sys 0.46