Add regex input validator

e.g:

```python
import re

class regex(object):
    """Regex input validator"""

    def __init__(self, pattern):
        self.pattern = pattern

    def __call__(self, value):
        if not re.match(self.pattern, value):
            message = 'Value does not match pattern: %s' % self.pattern
            raise MarshallingException(message)
        return value
```