$m in pre_do_shortcode_tag

The documentation for $m is available in the function that the pre_do_shortcode_tag filter gets called in, do_shortcode_tag():

/**
 * …
 * @param array $m {
 *     Regular expression match array.
 * 
 *     @type string $0 Entire matched shortcode text.
 *     @type string $1 Optional second opening bracket for escaping shortcodes.
 *     @type string $2 Shortcode name.
 *     @type string $3 Shortcode arguments list.
 *     @type string $4 Optional self closing slash.
 *     @type string $5 Content of a shortcode when it wraps some content.
 *     @type string $6 Optional second closing bracket for escaping shortcodes.
 * }