sed - output lines with one word

I believe I have found a potential bug in sed extended regular
expressions. I was trying to output lines containing only one word
from a text file. However, I found my expression to match more than it
should have.
Using the following command:
$sed -n -r '/^\s*[^\s]+\s*$/p' test.txt

Where test.txt conatins:
a
	 b
a b

Produces the following output:
 a
	 b
a b

In my opinion this is an unexpected behavior as this regular
expression should only print lines where there is zero or more
whitespace characters, followed by one or more non whitespace
characters, followed by zero or more whitespace characters, followed
by the end of the line.

I hope that this was of help.
Regards,
Josh McFarlane.

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