> I want to match the sequence "at least one letter" then "ple" 
> then "at least one letter". How would a regexp look like.?

"[:alpha:]+ple[:alpha:]+"

I think you should spend some time reading the Elisp manual section on regexps.

https://www.gnu.org/software/emacs/manual/html_node/elisp/Regular-Expressions.html

And maybe read up elsewhere about regexps:

https://www.oreilly.com/library/view/mastering-regular-expressions/0596528124/

Then you can help others with their questions about regexps...