private function update_dash_spacing_regex( $parenthetical, $parenthetical_space, $interval, $interval_space ) {
		// Mandatory UTF-8 modifier.
		$this->parenthetical_dash_spacing = "/
			(?:
				\s
				({$parenthetical})
				\s
			)
		/xu";

		// Mandatory UTF-8 modifier.
		$this->interval_dash_spacing = "/
			(?:
				(?<=\S)             # lookbehind assertion
				({$interval})
				(?=\S)              # lookahead assertion
			)
		/xu";

		$this->em_dash_replacement            = "{$interval_space}\$1\$2{$interval_space}"; // is this correct?
		$this->parenthetical_dash_replacement = "{$parenthetical_space}\$1\$2{$parenthetical_space}";
		$this->interval_dash_replacement      = "{$interval_space}\$1\$2{$interval_space}";
	}