Updated handling of '\b' when inside character class, made invalid

escapes an error.

The '\b' value refers to a word boundary normally, but refers to the
backspace ASCII value inside a character class. I updated
newEscapedNode() to deal with this. I also changed the behavior, so that
trying to escape any other value results in an error, instead of just
returning the character as-is.

postfixNode.go

@@ -1,5 +1,7 @@
 package main
 
+import "fmt"
+
 type NodeType int
 
 // This is a slice containing all escapable characters that have special meaning.
@@ -62,7 +64,7 @@ func newCharClassNode(nodes []postfixNode, negated bool) postfixNode {
 }
 
 // Creates a new escaped node - the given character is assumed to have been preceded by a backslash
-func newEscapedNode(c rune) postfixNode {
+func newEscapedNode(c rune, inCharClass bool) (postfixNode, error) {
 	toReturn := postfixNode{}
 	toReturn.startReps = 1
 	toReturn.endReps = 1
@@ -86,8 +88,13 @@ func newEscapedNode(c rune) postfixNode {
 		toReturn = newPostfixDotNode()
 		toReturn.except = append([]postfixNode{}, newPostfixNode(wordChars...))
 	case 'b', 'B':
-		toReturn.nodetype = ASSERTION
-		toReturn.contents = append(toReturn.contents, c)
+		if c == 'b' && inCharClass {
+			toReturn.nodetype = CHARACTER
+			toReturn.contents = append(toReturn.contents, rune(8))
+		} else {
+			toReturn.nodetype = ASSERTION
+			toReturn.contents = append(toReturn.contents, c)
+		}
 	case 'n': // Newline character
 		toReturn.nodetype = CHARACTER
 		toReturn.contents = append(toReturn.contents, '\n')
@@ -110,10 +117,13 @@ func newEscapedNode(c rune) postfixNode {
 		toReturn.nodetype = CHARACTER
 		toReturn.contents = append(toReturn.contents, rune(11))
 	default: // None of the above - append it as a regular character
+		if isNormalChar(c) { // Normal characters cannot be escaped
+			return postfixNode{}, fmt.Errorf("Invalid escape character.")
+		}
 		toReturn.nodetype = CHARACTER
 		toReturn.contents = append(toReturn.contents, c)
 	}
-	return toReturn
+	return toReturn, nil
 }
 
 // Creates and returns a postfixNode based on the given contents