+ artist_matches <pattern>
+ album_matches <pattern>
+ title_matches <pattern>
+ comment_matches <pattern>
+
+Takes an extended regular expression and matches iff the text of the
+corresponding tag of the audio file matches the pattern. If the tag
+is not set, the empty string is matched against the pattern.
+
+ year
+
+Takes a comparator ~ of the set {<, =, <=, >, >=, !=} and a number
+N. Matches iff the year tag Y of the audio file is a number that
+satisfies the condition Y ~ N. If the audio file has no year tag,
+or if the tag is not a number, the file does not match.