I have run several benchmarks, with the following scenarios:

(1) The benchmark emulates the current in_array check
(2) The benchmark builds a pattern using the array of characters and does a preg_match
(3) The benchmark builds a pattern using a class constant holding the complete alphabet
(4) The benchmark uses a pre-built pattern `/^[^A-Za-z0-9\+\/]+$/` and uses preg_match

These are the normalized results after running the suite a couple of times:

```
$ vendor/bin/athletic -p benchmark/Benchmarks/Validator/Encoding -b benchmark/bootstrap.php

Benchmarks\Validator\Encoding\Base64Event
    Method Name         Iterations    Average Time      Ops/second
    -----------------  ------------  --------------    -------------
    inArrayValidation: [25        ] [0.0473394680023] [21.12402]
    pregMatchList    : [25        ] [0.0000490379333] [20,392.37651]
    pregMatchAlphabet: [25        ] [0.0000236797333] [42,230.20540]
    pregMatchPattern : [25        ] [0.0000068378448] [146,244.90934]
```