Is your feature request related to a problem? Please describe.
I'm having several input plugins, for example tags aaa.* bbb.* ccc.* etc
I would like to apply record_modifier to everything but aaa.*

Describe the solution you'd like

<filter /(?!aaa).*/ >
   @type record_modifier
   ....

== or ==

<filter !aaa.**>
   @type record_modifier
   ....

Describe alternatives you've considered

explicitly filter for bbb.** ccc.** etc

Additional context
(?!aaa) doesn't work at the moment because every non-word character is escaped in
https://github.com/fluent/fluentd/blob/master/lib/fluent/match.rb#L113

I see 3 possible, easy to implement solutions:

1. allow regex with a / prefix, string must end with /
   <filter /some-regex/>

2. don't escape "(?!" and the corresponding ")"

3. allow pattern like
   !aaa.**

and create a regex (?!aaa).* from it