#' @param regex_pars An optional character vector of \link[=grep]{regular 
#'   expressions} to use for parameter selection. \code{regex_pars} can be used
#'   in place of \code{pars} or in addition to \code{pars}. Currently, all
#'   functions that accept a \code{regex_pars} argument ignore it for models fit
#'   using optimization.
