defer Regexp initialization to first use
defer Regexp initialization to first use

`benchstat` vs. master@588e8594267cca0d170ceda20342f7a883b80153:

    name             old time/op    new time/op    delta
    StrictEmpty-4      15.0ns ± 1%    14.7ns ± 1%   -1.68%  (p=0.000 n=7+10)
    StrictSingle-4     41.2µs ± 4%    33.9µs ± 2%  -17.81%  (p=0.000 n=10+10)
    StrictMany-4        125µs ± 1%     105µs ± 1%  -16.35%  (p=0.000 n=10+10)
    RelaxedEmpty-4     15.2µs ± 3%    10.0µs ± 2%  -34.30%  (p=0.000 n=10+10)
    RelaxedSingle-4    95.3µs ± 5%    58.2µs ± 3%  -38.87%  (p=0.000 n=10+9)
    RelaxedMany-4       247µs ± 1%     157µs ± 2%  -36.41%  (p=0.000 n=9+10)

    name             old alloc/op   new alloc/op   delta
    StrictEmpty-4       0.00B          0.00B          ~     (all equal)
    StrictSingle-4       364B ± 4%      187B ± 6%  -48.67%  (p=0.000 n=9+10)
    StrictMany-4         792B ± 6%      241B ±14%  -69.59%  (p=0.000 n=10+10)
    RelaxedEmpty-4      73.8B ± 2%     3.1B ±100%  -95.80%  (p=0.000 n=10+10)
    RelaxedSingle-4    2.43kB ± 4%    0.34kB ±13%  -85.89%  (p=0.000 n=9+10)
    RelaxedMany-4      5.02kB ± 3%    0.57kB ± 0%  -88.65%  (p=0.000 n=10+9)

    name             old allocs/op  new allocs/op  delta
    StrictEmpty-4        0.00           0.00          ~     (all equal)
    StrictSingle-4       2.00 ± 0%      2.00 ± 0%     ~     (all equal)
    StrictMany-4         4.00 ± 0%      4.00 ± 0%     ~     (all equal)
    RelaxedEmpty-4       0.00           0.00          ~     (all equal)
    RelaxedSingle-4      4.00 ± 0%      3.00 ± 0%  -25.00%  (p=0.000 n=10+10)
    RelaxedMany-4        8.00 ± 0%      5.00 ± 0%  -37.50%  (p=0.002 n=8+10)

mvdan/xurls