\tl_if_regex_match:nnTF  % warning on this line if
  % latex3_definitions_max_added_date < 2024-12-08
  { foo~bar }
  { foo }
  { bar }
  { baz }
\str_show:N  % warning on the following line if
  % latex3_definitions_max_added_date < 2020-08-20
  \c_sys_engine_format_str

.