create a filter using a regular expression to highlight duplicate words in a line

I would like to create a filter using a regular expression to highlight duplicate words in a line. I use the highlight package, which allows me to use regex to highlight certain words. Any ideas of what regex I could use for this purpose?

Classical regular expressions are not expressive enough to do this … they're not "stateful" in the way you need. You need "lookaround", which is in the class of "extended regexp"; emacs does not support it natively. If you have a specific, enumerated wordlist, you could construct a regex that uses lookaround to match each word separately … and as you can generally refer to existing-matched groups, something like… (foo|bar|baz|quux|…).+(?=\1) If you have an arbitrary word-list, you could do something like … ([a-zA-Z]{3,6}).*(?=\1) But that's going to match all sorts of silly things. You're probably looking to say something more like… \b([a-ZA-Z]+)\b.+(?=\1) … to match any "word-boundry"-delimited sequence of characters that shows up later in the string. Note at the bottom of [emacswiki.org/[…]/RegularExpression](https://www.emacswiki.org/emacs/RegularExpression) that with python available, you can use [visual-regexp-steroids](https://github.com/benma/visual-regexp-steroids.el/) to get access to sufficiently-expressive regexp. It's not clear to me if the python regexp lib can be used by highlight, tho.

Thank you so much for your detailed answer. I am going to test this solution and will revert to you.

https://github.com/ioah86/repetition\_error\_finder

Thanks to your direction, I got it to work. Many thanks,