validate stock query

src/main/java/com/danielbohry/stocks/controller/stock/StockController.java

@@ -14,8 +14,10 @@ import java.math.BigDecimal;
 import java.nio.charset.StandardCharsets;
 import java.util.List;
 import java.util.Objects;
+import java.util.regex.Pattern;
 
 import static java.time.LocalDateTime.now;
+import static java.util.Collections.emptyList;
 
 @Slf4j
 @RestController
@@ -26,8 +28,14 @@ public class StockController {
 
     private final StockService service;
 
+    private static final String ACCEPT = "^[a-zA-Z]+$";
+
     @GetMapping
-    public ResponseEntity<List<Quote>> getAll(@RequestParam(value = "q", required = false) String query) {
+    public ResponseEntity<List<Quote>> getAll(@RequestParam(value = "q") String query) {
+        if (query != null && !Pattern.matches(ACCEPT, query)) {
+            return ResponseEntity.ok(emptyList());
+        }
+
         List<Quote> response = service.get(query);
         return ResponseEntity.ok(response);
     }