Use an atom length of 2 for the regex filtering

[masklinn]

As it turns out a significant number of regexes have distinguishing atoms of length 2 rather than 3, leading to significant under-performing prefiltering using default settings e.g. when parsing sample 9997 (sort -u of sample file), the default setting prefilter from 633 to 61 regexes, of which the matching regex is number 50, leading to a lot of Regex::is_match.

Looking at the "extra" regexes, while they do have pretty long atoms those tend to be optional, the only required atoms are very short. By reducing the atom length to 2, the prefiltered set goes down to 20, of which the regex we're looking for is 14th. This cuts down the post-prefiltering filtering from 6µs to 2 (in addition to a 2µs prefiltering but that doesn't change much, it goes from 2.2 to 2.3).

This leads to a 15% perf increase on the benchmark, at no visible memory cost (maximum RSS and peak footprint are lost in noise), before:

Lines: 751580
Total time: 8.139572291s
10µs / line
        8.25 real         8.21 user         0.03 sys
            57655296  maximum resident set size
                   0  average shared memory size
                   0  average unshared data size
                   0  average unshared stack size
                3732  page reclaims
                   0  page faults
                   0  swaps
                   0  block input operations
                   0  block output operations
                   0  messages sent
                   0  messages received
                   0  signals received
                   0  voluntary context switches
                  85  involuntary context switches
         74982477832  instructions retired
         26557964231  cycles elapsed
            54461952  peak memory footprint

after:

Lines: 751580
Total time: 6.797529459s
9µs / line
        6.91 real         6.86 user         0.04 sys
            57802752  maximum resident set size
                   0  average shared memory size
                   0  average unshared data size
                   0  average unshared stack size
                3741  page reclaims
                   0  page faults
                   0  swaps
                   0  block input operations
                   0  block output operations
                   0  messages sent
                   0  messages received
                   0  signals received
                   0  voluntary context switches
                 154  involuntary context switches
         65652792138  instructions retired
         22207284899  cycles elapsed
            54478080  peak memory footprint