Throw error if \B is used in character class

regex/postfixNode.go

@@ -95,6 +95,9 @@ func newEscapedNode(c rune, inCharClass bool) (postfixNode, error) {
 			toReturn.nodetype = assertionNode
 			toReturn.contents = append(toReturn.contents, c)
 		}
+		if c == 'B' && inCharClass { // Invalid
+			return postfixNode{}, fmt.Errorf("word boundaries are not allowed in character class")
+		}
 	case 'n': // Newline character
 		toReturn.nodetype = characterNode
 		toReturn.contents = append(toReturn.contents, '\n')