Most Raygun providers include a feature to examine messages before they are sent, and then cancel the send if you don't care about them. This is useful for exceptions that you know aren't going to be solved, and don't want them to eat any of your monthly Raygun exception limit.
And here is the documentation for Raygun4Net (This is for ASP .Net. Other .Net platforms have different instructions): https://raygun.io/docs/languages/net/asp#modify-cancel-message
Raygun4Net also has the option to ignore exceptions based on status code, e.g. 404: https://raygun.io/docs/languages/net/asp#exclude-status-code
Let us know if you can't find the documentation for this feature for a specific platform.
As you will see, this feature works by providing a callback function to contain your cancel message logic, rather than providing a regex to the provider. This allows you to write any logic you want, and examine any parts of the message to determine if a message should be cancelled. You could certainly use regular expressions within your cancel logic.
If you were looking for a way to set up ignore rules in the Raygun app, rather than via the provider in your code, this is something we do not currently have. We might provide a way to do this from the app, but don't have an estimate of when.