Regex to match a word with + (plus) signs

The problem isn't with the plus character, that you've escaped correctly, but the \b sequence. It indicates a word boundary, which is a point between a word character (alphanumeric) and something else. Plus isn't a word character, so for \b to match, there would need to be a word character directly after the last plus sign.

\bC\+\+\b matches "Test C++Test" but not "Test C++ Test" for example. Try something like \bC\+\+\s if you expect there to be a whitespace after the last plus sign.

+ is a special character so you need to escape it

\bC\+\+(?!\w)

Note that we can't use \b because + is not a word-character.

Plus sign have special meaning so you will have to escape it with \. The same rule applies to these characters: \, *, +, ?, |, {, [, (,), ^, $,., #, and white space

UPDATE: the problem was with \b sequence