Benchmark: timing 10000000 iterations of exist, index, regex...
     exist: 50 wallclock secs (49.39 usr +  0.01 sys = 49.40 CPU)
     index: 61 wallclock secs (60.64 usr +  0.00 sys = 60.64 CPU)
     regex: 85 wallclock secs (83.18 usr +  0.03 sys = 83.21 CPU)