Rule 'HostRegexp({host:.+})' no longer works in Traefik v3

[ToshY]

Current docs denote the usage of the following rule:

      # use as "fallback" for any NON-registered services (with priority below normal)
      traefik.http.routers.error-pages-router.rule: HostRegexp(`{host:.+}`)

This rule works fine if my domain example.com is not yet accessible/deployed when using Traefik v2.11, and it will properly show the ghost template.

However when upgrading to Traefik v3.0 (v3.0.0-rc1 -> v3.0.0-rc5) it seems to no longer work and shows the default (traefik) 404 text:

---

I've been looking at the Traefik migration docs and it denotes that HostRegexp now uses so-called "Go regexp syntax", which I don't understand what's really changed.

[ToshY]

After a bit of digging in the docs, I've found the following section regarding "Rule syntax" which includes an example (luckily) with HostRegexp.

## Dynamic configuration
http:
  routers:
    Router-v3:
      rule: HostRegexp(`[a-z]+\\.traefik\\.com`)
      ruleSyntax: v3
    Router-v2:
      rule: HostRegexp(`{subdomain:[a-z]+}.traefik.com`)
      ruleSyntax: v2

So after seeing that, I tried HostRegexp(`.+`) instead, and that works.