Function is_valid_key 

pub fn is_valid_key(key: &str) -> bool

Whether key is usable as-is as a BibTeX / biblatex citation key: non-empty and made only of letters, digits and -_:.+/. An explicit --key is checked with this and refused otherwise, rather than silently rewritten: smith, 2020 would otherwise end the key at the comma and shift every field after it (review of #622).