Use POSIX bracket expressions in raw regex literals

[Forthoney]

We should consider using POSIX bracket expressions instead of raw regex patterns where applicable since POSIX bracket expressions better express the intention of a regex pattern.
Take FIELD_VALUE = /[^\000-\037]*/ in connection.rb for example. Expressing /[^\000-\037]*/ as [[:print:]]* does a better job of expressing that a field value must be a printable value.

[ioquatix]

I'm open to improvements, but IIRC the reason why I did this was to replicate exactly what was in the RFC. In theory, it's easy to see that the definition in this code matches the RFC just by visual comparison.