that works

Hmm. I swear that I have early dementia.
When I tested it out I used a 5-letter-word.
But when I posted, for some reason I changed it to a 6-letter example (“letter”), which would not work the way I wrote it before.

So maybe change my prior example to:

find: (?=\b\w{6}\b)(?i)l.tt.r
mode: regular expression

I think you can see how you might adjust it to use any specific number of characters…

So the use of “word” implies that you define a word appropriately. A common definition is characters from the English set A,B,C,…,Z or lowercase, the digits 0,1,2,…9, and the underscore character. If this is acceptable then for your first question an expression to search on would be:

find: \b\w{5}\b
mode: regular expression

When coupled with your second condition, I might go with a search expression like this one, for the word letter with some letters you don’t know:

find: (?=\b\w{5}\b)(?i)l.tt.r
mode: regular expression

The . would represent the letters you don’t know.