Adds a hyphen to the product SKU right after three characters starting from the left

functions.php

// Adds a hyphen to the product SKU right after three characters starting from the left
add_filter ( 'woocommerce_product_search_indexer_filter_content', 'example_woocommerce_product_search_indexer_filter_content', 10, 3 );
function example_woocommerce_product_search_indexer_filter_content( $content, $context, $post_id ) {
	if ( $context === 'post_content' ) {
		$product = wc_get_product( $post_id );
		if ( $product ) {
			$product_sku = $product->get_sku();
			$alternative_product_sku = substr_replace( $product_sku, '-', 3, 0 );
			$content .= ' ' . $alternative_product_sku;
		}
	}
	return $content;
}