Benchmark: timing 250000 iterations of foreachmapsplit, greprxmapsplit, mapsplit...

foreachmapsplit:  4 wallclock secs ( 3.46 usr +  0.00 sys =  3.46 CPU) @ 72254.34/s (n=250000)
greprxmapsplit:  4 wallclock secs ( 3.24 usr +  0.00 sys =  3.24 CPU) @ 77160.49/s (n=250000)
  mapsplit:  4 wallclock secs ( 3.24 usr +  0.00 sys =  3.24 CPU) @ 77160.49/s (n=250000)