A valid variable name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. As a regular expression, it would be expressed thus: '[a-zA-Z_x7f-xff][a-zA-Z0-9_x7f-xff]*'

Technically, you can get around this in two different ways - variable varaibles and the more complex notation with curly braces. He points to the compact function for proof that they're set.