Source

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

protected function parse_search( &$q ) {
		global $wpdb;

		$search = '';

		// Added slashes screw with quote grouping when done early, so done later.
		$q['s'] = stripslashes( $q['s'] );
		if ( empty( $_GET['s'] ) && $this->is_main_query() ) {
			$q['s'] = urldecode( $q['s'] );
		}
		// There are no line breaks in <input /> fields.
		$q['s']                  = str_replace( array( "\r", "\n" ), '', $q['s'] );
		$q['search_terms_count'] = 1;
		if ( ! empty( $q['sentence'] ) ) {
			$q['search_terms'] = array( $q['s'] );
		} else {
			if ( preg_match_all( '/".*?("|$)|((?<=[\t ",+])|^)[^\t ",+]+/', $q['s'], $matches ) ) {
				$q['search_terms_count'] = count( $matches[0] );
				$q['search_terms']       = $this->parse_search_terms( $matches[0] );
				// If the search string has only short terms or stopwords, or is 10+ terms long, match it as sentence.
				if ( empty( $q['search_terms'] ) || count( $q['search_terms'] ) > 9 ) {
					$q['search_terms'] = array( $q['s'] );
				}
			} else {
				$q['search_terms'] = array( $q['s'] );
			}
		}

		$n                         = ! empty( $q['exact'] ) ? '' : '%';
		$searchand                 = '';
		$q['search_orderby_title'] = array();

		/**
		 * Filters the prefix that indicates that a search term should be excluded from results.
		 *
		 * @since 4.7.0
		 *
		 * @param string $exclusion_prefix The prefix. Default '-'. Returning
		 *                                 an empty value disables exclusions.
		 */
		$exclusion_prefix = apply_filters( 'wp_query_search_exclusion_prefix', '-' );

		foreach ( $q['search_terms'] as $term ) {
			// If there is an $exclusion_prefix, terms prefixed with it should be excluded.
			$exclude = $exclusion_prefix && ( substr( $term, 0, 1 ) === $exclusion_prefix );
			if ( $exclude ) {
				$like_op  = 'NOT LIKE';
				$andor_op = 'AND';
				$term     = substr( $term, 1 );
			} else {
				$like_op  = 'LIKE';
				$andor_op = 'OR';
			}

			if ( $n && ! $exclude ) {
				$like                        = '%' . $wpdb->esc_like( $term ) . '%';
				$q['search_orderby_title'][] = $wpdb->prepare( "{$wpdb->posts}.post_title LIKE %s", $like );
			}

			$like      = $n . $wpdb->esc_like( $term ) . $n;
			$search   .= $wpdb->prepare( "{$searchand}(({$wpdb->posts}.post_title $like_op %s) $andor_op ({$wpdb->posts}.post_excerpt $like_op %s) $andor_op ({$wpdb->posts}.post_content $like_op %s))", $like, $like, $like );
			$searchand = ' AND ';
		}

		if ( ! empty( $search ) ) {
			$search = " AND ({$search}) ";
			if ( ! is_user_logged_in() ) {
				$search .= " AND ({$wpdb->posts}.post_password = '') ";
			}
		}

		return $search;
	}