simplify regular expression patterns
simplify regular expression patterns

`benchstat` vs. master@588e8594267cca0d170ceda20342f7a883b80153:
    name             old time/op    new time/op    delta
    StrictEmpty-4      15.0ns ± 1%    15.1ns ± 0%   +1.15%  (p=0.001 n=7+7)
    StrictSingle-4     41.2µs ± 4%    41.0µs ± 2%     ~     (p=0.905 n=10+9)
    StrictMany-4        125µs ± 1%     126µs ± 2%     ~     (p=0.436
    n=10+10)
    RelaxedEmpty-4     15.2µs ± 3%    15.1µs ± 5%     ~     (p=0.089
    n=10+10)
    RelaxedSingle-4    95.3µs ± 5%    93.9µs ± 3%     ~     (p=0.436
    n=10+10)
    RelaxedMany-4       247µs ± 1%     243µs ± 3%   -1.71%  (p=0.004 n=9+10)

    name             old alloc/op   new alloc/op   delta
    StrictEmpty-4       0.00B          0.00B          ~     (all equal)
    StrictSingle-4       364B ± 4%      349B ± 9%     ~     (p=0.197 n=9+10)
    StrictMany-4         792B ± 6%      746B ±12%     ~     (p=0.222
    n=10+10)
    RelaxedEmpty-4      73.8B ± 2%     6.1B ±100%  -91.73%  (p=0.000
    n=10+10)
    RelaxedSingle-4    2.43kB ± 4%    2.02kB ± 0%  -16.98%  (p=0.000 n=9+8)
    RelaxedMany-4      5.02kB ± 3%    3.92kB ± 4%  -22.00%  (p=0.000
    n=10+10)

    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