+/** @brief Match a track against a rgeexp
+ * @param dl Length of directory part of track
+ * @param track Track name
+ * @param tl Length of track name
+ * @param re Regular expression or NULL
+ * @return Nonzero on match
+ *
+ * @p tl is the total length of @p track, @p dl is the length of the directory
+ * part (the index of the final "/"). The subject of the regexp match is the
+ * basename, i.e. the part after @p dl.
+ *
+ * If @p re is NULL then always matches.
+ */