This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
DOS 3.3 filenames may be up to 30 characters long, and must conform to the following restrictions: a. The first character must have an ASCII code value greater than 63. This excludes numerals, SPACE, and most punctuation. The "@" character is allowed as well as letters, and some special chars such as "]", "~", "_", and a few others. b. Commas and colons may not be used at all. Apart from these restrictions, anything goes. All kinds of characters, including uppercase, lowercase, numbers, symbols, and CONTROL characters can be used after the first character. By: Andy McFadden