FileSystem: Improve regex by only matching once instead of 3x.

shared/util/codeql/util/FileSystem.qll

@@ -84,6 +84,17 @@ module Make<InputSig Input> {
      */
     string getAbsolutePath() { result = super.getAbsolutePath() }
 
+    /**
+     * Holds if either,
+     * - `part` is the base name of this container and `i = 1`, or
+     * - `part` is the stem of this container and `i = 2`, or
+     * - `part` is the extension of this container and `i = 3`.
+     */
+    cached
+    private predicate splitAbsolutePath(string part, int i) {
+      part = this.getAbsolutePath().regexpCapture(".*/(([^/]*?)(?:\\.([^.]*))?)", i)
+    }
+
     /**
      * Gets the base name of this container including extension, that is, the last
      * segment of its absolute path, or the empty string if it has no segments.
@@ -101,9 +112,7 @@ module Make<InputSig Input> {
      * <tr><td>"//FileServer/"</td><td>""</td></tr>
      * </table>
      */
-    string getBaseName() {
-      result = this.getAbsolutePath().regexpCapture(".*/(([^/]*?)(?:\\.([^.]*))?)", 1)
-    }
+    string getBaseName() { this.splitAbsolutePath(result, 1) }
 
     /**
      * Gets the extension of this container, that is, the suffix of its base name
@@ -128,9 +137,7 @@ module Make<InputSig Input> {
      * <tr><td>"/tmp/x.tar.gz"</td><td>"gz"</td></tr>
      * </table>
      */
-    string getExtension() {
-      result = this.getAbsolutePath().regexpCapture(".*/([^/]*?)(\\.([^.]*))?", 3)
-    }
+    string getExtension() { this.splitAbsolutePath(result, 3) }
 
     /** Gets the file in this container that has the given `baseName`, if any. */
     File getFile(string baseName) {
@@ -183,9 +190,7 @@ module Make<InputSig Input> {
      * <tr><td>"/tmp/x.tar.gz"</td><td>"x.tar"</td></tr>
      * </table>
      */
-    string getStem() {
-      result = this.getAbsolutePath().regexpCapture(".*/([^/]*?)(?:\\.([^.]*))?", 1)
-    }
+    string getStem() { this.splitAbsolutePath(result, 2) }
 
     /**
      * Gets a URL representing the location of this container.