Andrey Semashev via Boost writes:

Boost.Log can be configured to use std::regex or Boost.Xpressive as an alternative to Boost.Regex.

https://www.boost.org/doc/libs/1_81_0/libs/log/doc/html/log/installation/con...

Boost.Regex is the default because it provides the best performance. I'm not planning to change this.

I don't think there would be strong motivation to change this in Log since it's a leaf library (i.e., no other Boost library depends on it). But we can make this configurable in our package if/when there is interest.