Benchmark: timing 1000 iterations of foreach_middle, grep_middle, hash_middle...
foreach_middle:  1 wallclock secs ( 0.80 usr +  0.00 sys =  0.80 CPU) @ 1250.00/s (n=1000)
grep_middle:  3 wallclock secs ( 3.09 usr +  0.00 sys =  3.09 CPU) @ 323.62/s (n=1000)
hash_middle: 30 wallclock secs (26.40 usr +  0.02 sys = 26.42 CPU) @ 37.85/s (n=1000)