According to the tagger script grammar [1]  and the actual implementation a function name can consist of the characters [a-zA-Z0-9_], without restriction on placement.

Thus the following are all valid function names: $_, $123, $_a, $_1a

But the syntax highlighting assumes function names must start with [a-zA-Z].

[1] https://github.com/metabrainz/picard/blob/master/picard/script/parser.py#L202-L212