Fix issue with named backrefs; add needed import

Author: joefarebrother

java/ql/lib/semmle/code/java/regex/RegexTreeView.qll

@@ -258,6 +258,9 @@ class RegExpQuantifier extends RegExpTerm, TRegExpQuantifier {
     result.occursInRegex(re, start, part_end)
   }
 
+  /** Holds if this term may match zero times. */
+  predicate mayBeEmpty() { maybe_empty = true }
+
   /** Holds if this term may match an unlimited number of times. */
   predicate mayRepeatForever() { may_repeat_forever = true }
 

java/ql/lib/semmle/code/java/regex/regex.qll

@@ -112,7 +112,7 @@ abstract class RegexString extends StringLiteral {
     this.getChar(start + 1) = "k" and
     this.getChar(start + 2) = "<" and
     end = min(int i | i > start + 2 and this.getChar(i) = ">") + 1 and
-    name = this.getText().substring(start + 3, end - 2)
+    name = this.getText().substring(start + 3, end - 1)
   }
 
   private predicate numberedBackreference(int start, int end, int value) {

java/ql/lib/semmle/code/java/security/performance/ReDoSUtilSpecific.qll

@@ -4,6 +4,7 @@
  */
 
 private import java
+import semmle.code.FileSystem
 import semmle.code.java.regex.RegexTreeView
 
 /**