It looks like the main issue with your regex expression is the capital ‘R’. In doing some testing, I found out that the search does not respect the capitalized letters and only works with lower case letters in Regex. If you change it to /ra.*/
then you should pick up the users that you are looking for. I have a topic discussion started on this at the link below.
Additionally, it looks like you have a typo in your regex patter above. You have /\bRa./*
when I believe you meant /\bRa.*/
Also, it looks like Elasticsearch does not handle anchor tags, per the last paragraph in this document, and rather always anchors to the start of the word.
Discussion on issue: