Find the first letter of each word with a double letter in a text

I was playing Planetarium (https://www.beholder.co.uk/planetarium/) and one of the last puzzles was to find words made up from the first letter, of each word with a double letter, in a given text. As there was a lot of text to sift through. I decided a programmatic solution might be helpful.

Regular Expressions were the obvious solution. A regular expression (regex or regexp for short) is a special text string for describing a search pattern. You can think of regular expressions as wildcards on steroids. You are probably familiar with wildcard notations such as *.txt to find all text files in a file manager. The regex equivalent is ^.*\.txt$. (from: https://www.regular-expressions.info/ )

There are many online regexp generator tools. I chose: https://regexr.com/

The expression I came up with to do the task was:
\b\w{1}(?=\w*([a-zA-Z])\1\w*)
(paste that into the expression box)

You can modify the text below if desired.

Voila. The first letter of each word with a double letter in a text.