message: Feature#586: Check if course exists
  Added RegEx for checking if the id only contains characters from a-f. Also added check, if the length of th id is 24 characters long.
diff: @@ -219,9 +219,9 @@ export class CourseController {
  */
  @Get('/:id')
  async getCourse(@Param('id') id: string, @CurrentUser() currentUser: IUser) {
  - const regex = '/[g-z]/'; // Check if id only contains characters from a-f and numbers
  + const regex = /[g-z]/gi; // Check if id only contains characters from a-f
  - if (id.match(regex) !== null || id.length > 24) {
  + if (id.search(regex) !== -1 || id.length === 24) {
  throw new NotFoundError();
  }
  