Fix error that occurs when parsing expression containing '+' character

[carrieedwards]

We are seeing errors occurring when an expression that contains regexp with the '+' character. For example:

alias(testMetric[0-9]+.Metric, 'name')

The error that is being thrown is unexpected character.

The Graphite docs on wildcard expressions do not explicitly mention the '+' character as unsupported, it does not throw an error and seems to process it. This character should be supported in the parser in to prevent throwing the error.