Change \B to be the opposite of \b.

Also update docs.
Fixes https://github.com/boostorg/regex/issues/123.

include/boost/regex/v5/perl_matcher_common.hpp

@@ -471,6 +471,11 @@ bool perl_matcher<BidiIterator, Allocator, traits>::match_word_boundary()
 template <class BidiIterator, class Allocator, class traits>
 bool perl_matcher<BidiIterator, Allocator, traits>::match_within_word()
 {
+   bool b = !match_word_boundary();
+   if(b)
+      pstate = pstate->next.p;
+   return b;
+   /*
    if(position == last)
       return false;
    // both prev and this character must be m_word_mask:
@@ -492,6 +497,7 @@ bool perl_matcher<BidiIterator, Allocator, traits>::match_within_word()
       }
    }
    return false;
+   */
 }
 
 template <class BidiIterator, class Allocator, class traits>