Allow @/$ prefix in Regexp's named captures
fluentd hits the problem that Regexp's named captures doesn't allow @ prefix.
I assume this limitation is for auto variable assign feature.
In fluentd, there is no auto assign use case so we want to use @/$ prefix.
@ is used for Elastic related products and $ is used for MongoDB.
Currently, we need to add extra routine to convert foo to @foo / $foo after parsed the data.
It makes performance slower and causes complicated configuration.
I'm not sure the best approach but we want command line option or something to allow @/$ prefix in named captures.