Benchmark: timing 100000 iterations of if_regex, if_substr, regex, substr...
  if_regex:  3 wallclock secs ( 2.50 usr +  0.00 sys =  2.50 CPU) @ 40000.00/s (n=100000)
 if_substr:  2 wallclock secs ( 2.47 usr +  0.00 sys =  2.47 CPU) @ 40485.83/s (n=100000)
     regex:  5 wallclock secs ( 4.88 usr +  0.00 sys =  4.88 CPU) @ 20491.80/s (n=100000)
    substr:  4 wallclock secs ( 2.57 usr +  0.00 sys =  2.57 CPU) @ 38910.51/s (n=100000)
             Rate     regex    substr  if_regex if_substr
regex     20492/s        --      -47%      -49%      -49%
substr    38911/s       90%        --       -3%       -4%
if_regex  40000/s       95%        3%        --       -1%
if_substr 40486/s       98%        4%        1%        --