It seems character classes [...] are not working in MP3Tag's implementation of regular expressions, as
$regexp(%_tag%,(ID3\w*).,$1) matches ID3v1
but $regexp(%_tag%,(ID3[\w]).,$1) only matches ID3(should match ID3v1) $regexp(%_tag%,(ID3[^ ]).*,$1) only matches ID3(should match ID3v1)