implement RegExpWordBoundary in RegexTreeView

python/ql/lib/semmle/python/RegexTreeView.qll

@@ -525,6 +525,13 @@ private int toHex(string hex) {
   result = 15 and hex = ["f", "F"]
 }
 
+/**
+ * A word boundary, that is, a regular expression term of the form `\b`.
+ */
+class RegExpWordBoundary extends RegExpEscape {
+  RegExpWordBoundary() { this.getUnescaped() = "b" }
+}
+
 /**
  * A character class escape in a regular expression.
  * That is, an escaped charachter that denotes multiple characters.