Fix parser tokenizer bug (alignment operator).

Alignment operator was too "loose", i.e. in the query
[word='the'] =test=> _, "='the'] =test=>" was seen as an
alignment operator.

Now disallowed several special characters inside the operator
regular exoression to prevent this problem. Relation types
should generally consist of normal characters, not quotes, spaces,
brackets, etc. If really needed, we could add an option to put
regexes between =/.../=> , but that's probably not necessary.