Whitespace in CCCs in extended syntax causes trap when matching

The following traps in `Processor.cycle()`, due to the unescaped space inside the custom character class:

```swift
try! Regex(compiling: #"(?xx)[ \t]+"#).matchWhole(" \t ")
```

Removing the space (`[\t]`) or escaping it (`[\ \t]`) resolves the crash.