webpack serve giving "Invalid regular expression" error

I was running Node 8.9.1 instead of the latest stable version, which was causing the problems.

nvm install stable
nvm use stable

In my case (Node 12, Webpack 5.3.1), updating webpack-cli to 4.2.0 solved this error

I'm having the same issue just today. I solve it with updating my node version to the latest stable version

sudo npm cache clean -f
sudo npm install -g n
sudo n stable