Benchmark: timing 1000000 iterations of double_grep, grep_and, lookahead_grep...
double_grep: 11 wallclock secs ( 9.06 usr +  0.00 sys =  9.06 CPU) @ 110350.92/s (n=1000000)
  grep_and:  8 wallclock secs ( 8.70 usr +  0.00 sys =  8.70 CPU) @ 114902.91/s (n=1000000)
lookahead_grep: 21 wallclock secs (20.31 usr +  0.00 sys = 20.31 CPU) @ 49231.98/s (n=1000000)