Strings that match /^\w+$/ are quite common. For example many authentication systems use that pattern to validate usernames. I’m wondering if a term exists that identifies this kind of strings.
I’ve been thinking of the term “alphanumeric” but is a string alphanumeric if it contains an underscore?
I do not think that it is a good idea to give it a potentially confusing name. Alphanumeric could be misunderstood. If you need to refer to it, for example in your documentation, define it with an unique name like ‘username_pattern’, and refer to that definition of yours whenever you talk about it.