Your regex is syntactically sound, at least if by "information on the internet", you mean the documentation that comes with Perl for regexes, like perlre.

But there seems to be a logic error in your regular expression. You say "all 3-letter words that occur more than once". Maybe you can give us some examples what should match and what shouldn't.

Your regular expression, as is, never matches "words", because there is no "word" (\w) that can have a "word boundary" (\b) in its middle. Maybe you want something else to appear between the two "words"?

Consider the following data and tell us which "words" on the lines should match, and which ones shouldn't.

foofoo
foo foo
foo bar foo
foo bar foo bar
foooof
foo oof
foo and bar
foo foo and and bar bar
foo and bar foo and bar
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