How do I create a regular expression to match a word misspelling the original case sensitivity?

Question

I want to discover wrong spelling of "FooBar" in sentence:

"This is a 'FooBar' example where I should match different spelling of fooBar such as: foobar, FOOBAR or even fOoBaR but not foobarS!"

In this sentence, I would like to match words (in order): fooBar, foobar, FOOBAR, fOoBaR and not: FooBar (correct spelling), foobarS (not the same word)

Is there an existing solution using Perl Regular Expression? This is intended to be used with grep -P

Thanks

Answer 1

echo "$str" | grep -P '\b(?!FooBar)(?i:foobar)\b'

Answer 2

cat foobar.txt | grep -P '(?!FooBar)[fF][oO][oO][bB][aA][rR]'

Answer 3

(?<=^|\W)(?!FooBar)(?i)foobar(?=\W|$)