Hi @ancesar,
Welcome to the 
community support forums !!
I was actually working on the timestamp conversion this week and found like you that either can use the Transformation function OR can also use Overwrite function.
If the Transformation did not help you then please have a look at this post where I created a small screencast on how to convert the timestamp into YYYY-MM-DD
For your other question regarding the REGEX, well I will be honest that I do not have that much expertise on this topic. But if you think this might be a bug as you already have done testing, then I can recommend opening this as an issue on our OSS GitHub repository at this link