code: private boolean isSockJsSpecialChar(char ch) {
          return ch <= '\u001F'
                  || ch >= '\u200C' && ch <= '\u200F'
                  || ch >= '\u2028' && ch <= '\u202F'
                  || ch >= '\u2060' && ch <= '\u206F'
                  || ch >= '\uFFF0'
                  || ch >= '\uD800' && ch <= '\uDFFF';
      }
docs: See `escapable_by_server` variable in the SockJS protocol test suite.