sanitizing expression with regex pattern

Author: akulk022

framework/src/main/java/org/checkerframework/framework/util/JavacParseUtil.java

@@ -9,6 +9,7 @@
 import java.io.IOException;
 import java.net.URI;
 import java.util.Collections;
+import java.util.regex.Pattern;
 import javax.tools.JavaCompiler;
 import javax.tools.JavaFileObject;
 import javax.tools.SimpleJavaFileObject;
@@ -26,6 +27,16 @@ public class JavacParseUtil {
   /** Creates a JavacParseUtil. */
   public JavacParseUtil() {}
 
+  // Pattern to reject clearly invalid expressions before parsing
+  private static final Pattern EXPRESSION_GATE =
+      Pattern.compile(
+          "^(?!.*;)\\s*"
+              + "(?!\\[?\\s*error\\s+for\\s+expression:)"
+              + "(?:(?!(?:final\\s+)?(?:byte|short|int|long|float|double|boolean|char|var)\\b.*=).)*"
+              + "(?:(?!(?:if|switch|for|while|do|try|catch|finally|return|throw|break|continue|class|interface|enum)\\b).)*"
+              + ".+\\S.*$",
+          Pattern.CASE_INSENSITIVE | Pattern.DOTALL);
+
   /**
    * Parses the given Java expression string and returns it as a {@link ExpressionTree} using the
    * {@code javac} compiler API.
@@ -36,16 +47,13 @@ public JavacParseUtil() {}
    * @throws RuntimeException if parsing fails or the expression cannot be found in the AST
    */
   public static ExpressionTree parseExpression(String expressionSource) {
-    // This method works by embedding the expression in a dummy class and variable declaration, then
-    // parsing the resulting source to extract the expression tree.
-    //
-    // For example, the input {@code "1 + 2"} is transformed into:
-    //   class Dummy { Object expression = 1 + 2; }
-    //
-    // The initializer of the {@code expression} field is then extracted and returned.
+    String sanitized = expressionSource.replaceAll("#num(\\d+)", "\\$num$1").trim();
+
+    // Quick pre-check to skip obvious non-expressions
+    if (!EXPRESSION_GATE.matcher(sanitized).matches()) {
+      throw new RuntimeException("Not a valid Java expression: " + expressionSource);
+    }
 
-    // Embed the expression inside a dummy class and variable declaration.
-    String sanitized = expressionSource.replaceAll("#num(\\d+)", "\\$num$1");
     String dummySource = "class Dummy { Object expression = " + sanitized + "; }";
 
     // Obtain the system Java compiler.