Not word boundary - \b vs. NOT \B are not the same

[anonymous]

I am reposting this from another source.

In theory, \B should match everywhere \b doesn't.

In boost regex, this is not the case at the beginning nor end of string.

Below is a list of test results from a few Perl-like engines. Apparently, this was fixed in Perl v5.22 (below shows v5.20). The only engines that seem to handle this correctly is PHP and ECMAScript.

---

Target      = '   ssssssssssssss '
Replacement = '<>'

==================================================
PHP 7.03

\b      = '   <>ssssssssssssss<> '
\B      = '<> <> <> s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s <>'
(?!\b)  = '<> <> <> s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s <>'
(?<!\b) = '<> <> <> s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s <>'
(?!\B)  = '   <>ssssssssssssss<> '
(?<!\B) = '   <>ssssssssssssss<> '


=======================================
Perl 5.20

\b      = '   <>ssssssssssssss<> '
\B      = '<> <> <> s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s '
(?!\b)  = '<> <> <> s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s <>'
(?<!\b) = '<> <> <> s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s <>'
(?!\B)  = '   <>ssssssssssssss<> '
(?<!\B) = '   <>ssssssssssssss<> '

========================================
Boost 1.64

\b      = '   <>ssssssssssssss<> '
\B      = ' <> <> s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s '
(?!\b)  = '<> <> <> s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s <>'
(?<!\b) = '<> <> <> s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s <>'
(?!\B)  = '<>   <>ssssssssssssss<> <>'
(?<!\B) = '<>   <>ssssssssssssss<> <>'

=====================================
JavaScript

\b      = '   <>ssssssssssssss<> '
\B      = '<> <> <> s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s <>'
(?!\b)  = '<> <> <> s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s<>s <>'
(?!\B)  = '   <>ssssssssssssss<> '