All variables, functions, and object properties are named with identifiers. ActionScript identifiers must be composed according to the following rules:
Identifiers must start with a letter, underscore, or dollar sign (not a number).
Identifiers cannot be identical to reserved words.
For safety, identifiers should include only letters (A-Z or a-z), numbers, underscores, and dollar signs. Be especially careful never to use spaces, periods, backslashes, or other punctuation in identifiers. However, most Unicode characters are, in fact, permitted in identifiers, according to the rules outlined in section 3.16 of the Unicode standard. (See http://www.unicode.org/unicode/standard/standard.html).
A character in an identifier cannot be specified with a Unicode escape sequence (\uxxxx). (This is a departure from ECMA-262, v3, which allows escape sequences in identifiers.)
See the next section on case-sensitivity for more caveats about identifiers.
Though not strictly required, it's also good practice to follow these rules when composing movie clip instance names, symbols, symbol export identifiers, frame labels, and layer names. For information on using standardized suffixes in identifier names (such as _mc to indicate a movie clip) see Table 2-1 in Chapter 2.