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Appendix. Variable Naming Rules

Here are the rules for variable names (also called identifiers):

• They must begin with either a letter or an underscore character. Following that, you can use an unlimited sequence of letters, numbers, or underscore characters.
• The letters can be uppercase or lowercase, and case matters. In other words, Ax is not the same as aX.
• The numbers can be any of the digit characters from 0 to 9 inclusive.

Aside from letters, numbers, and the underscore character, no other characters can be used. Spaces, punctuation, and other characters are not allowed in variable names:

~ ` ! @ # $ % ^ & * ( ) ; - : " ' < > , . ? / { } [ ] + = /

The only special character that’s allowed is the underscore character. In case you don’t ...