WP_Rewrite::add_rewrite_tag( string $tag, string $regex, string $query )

Source

File: wp-includes/class-wp-rewrite.php

public function add_rewrite_tag( $tag, $regex, $query ) {
		$position = array_search( $tag, $this->rewritecode, true );
		if ( false !== $position && null !== $position ) {
			$this->rewritereplace[ $position ] = $regex;
			$this->queryreplace[ $position ]   = $query;
		} else {
			$this->rewritecode[]    = $tag;
			$this->rewritereplace[] = $regex;
			$this->queryreplace[]   = $query;
		}
	}