What can a regular expression do?

Question

Regular Expressions

There is a line
"word water porridge word four water"
is it possible to use a regular expression to find out to select only those words that do not repeat?

Answer 1

No. The task of regular expressions is to find a match according to the pattern. Searching for non-recurring ones is a completely different algorithm.

Answer 2

As an option, put each word in a separate array key and remove duplicates from it using a suitable method (because I don’t know what language you have there).
The first thing that came to mind...

Answer 3

A regular can find a repetition, but this task is not for regulars, because you can’t do without a loop and conditions to search for nesting.
You can try
https://regex101.com/r/JxyfqR
As you can see, the first repeated "word" was found. But to search for the second repetition, it is more difficult or impossible to write.