What are valid Liquid variable names?

Can Liquid variable names start with a number? I’m getting the following error, where 2a is the name of a variable, that goes away when I rename the variable to a2:

Liquid Warning: Liquid syntax error (line 9): Expected end_of_string but found id in "2a" in /_layouts/post.html

The (presumably) offending code:

{% if 2a %} whatever {% endif %}