Regular expressions fail to match escaped operator \+ when enclosed with \b

I have a string "foo + bar". I attempt to write a regular expression matching the free-standing "+" operator.
However, my initial attempt QRegularExpression("\\b\\+
b") didn't succeed — it fails to match to the said string.

If I replace "
+" with "a", it works. If I replace "
b" with " " (space), it works (but does different thing than I wanted). So apparently both parts are fine on their own, they fail only when composed together.

Please consider the following source code snippet:

	const QString textA = "foo a bar";
	const QRegularExpression regexpA("\\ba\\b");
	const QString textPlus = "foo + bar";
	const QRegularExpression regexpPlus("\\+");
	const QRegularExpression regexpPlusEnclosed("\\b\\+\\b");
	assert(regexpA.globalMatch(textA).hasNext()); // OK
	assert(regexpPlus.globalMatch(textPlus).hasNext()); // OK
	assert(regexpPlusEnclosed.globalMatch(textPlus).hasNext()); // FAILURE

I believe all three matches should succeed, while the third one fails.