Add a regex example

app.py

@@ -17,7 +17,9 @@ def main():
         gr.Markdown(DESCRIPTION)
         search_box = gr.Textbox(
             label='Keywords',
-            placeholder='You can search for titles with regular expressions')
+            placeholder=
+            'You can search for titles with regular expressions. e.g. (?<!sur)face'
+        )
         case_sensitive = gr.Checkbox(label='Case Sensitive')
         search_button = gr.Button('Search')
         names_with_link = gr.CheckboxGroup(choices=[