A substring filter: attr=initial*any*...*final, with at least one piece present.
attr=initial*any*...*final
Optional
Ordered middle pieces (between *s).
*
The attribute description.
The trailing anchor (the text after the last *), if any.
The leading anchor (the text before the first *), if any.
Discriminator.
A substring filter:
attr=initial*any*...*final, with at least one piece present.