Source

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

protected function parse_search_terms( $terms ) {
		$strtolower = function_exists( 'mb_strtolower' ) ? 'mb_strtolower' : 'strtolower';
		$checked    = array();

		$stopwords = $this->get_search_stopwords();

		foreach ( $terms as $term ) {
			// Keep before/after spaces when term is for exact match.
			if ( preg_match( '/^".+"$/', $term ) ) {
				$term = trim( $term, "\"'" );
			} else {
				$term = trim( $term, "\"' " );
			}

			// Avoid single A-Z and single dashes.
			if ( ! $term || ( 1 === strlen( $term ) && preg_match( '/^[a-z\-]$/i', $term ) ) ) {
				continue;
			}

			if ( in_array( call_user_func( $strtolower, $term ), $stopwords, true ) ) {
				continue;
			}

			$checked[] = $term;
		}

		return $checked;
	}