Many templating languages support filtering individual interpolated values:

"Hello, {name | capitalize}!"
//=> with {name: "john doe"} => "Hello, John Doe!"

pupa supports transforming all values, but not individual ones. The regex to match items could be modified to:

/{(\d+|[a-z$_][\w\-$]*?(?:\.[\w\-$]*?)*?)(?: \| [^ }]+)*}/gi

This would match an arbitrary number of filters separated by |. (regex101)

Filters would just be an object of functions passed to pupa and would be applied in order from left to right. Filters in the template without a match could have the same semantics as a missing value (either get ignored or throw, perhaps with a new MissingFilterError).

Related, some further improvements to the regex could be supporting arbitrary whitespace and characters:

/{\s*([^\s}|]+)\s*(?:\|\s*[^}]+\s*)*}/gi