The ExpandCharacterClass(ccl) command expands character class expressions as described in the following section. These expressions are used to describe sets of characters in wildcard and regular expression patterns, but may be used for other purposes as well.

If the string ccl is a valid character class expression, then a string, with at most 255 characters, is returned that denotes literally the expressed character class.

In a character class expression, most characters denote themselves literally. For example, the character class expression  denotes the character class consisting of the characters ,  and , which is represented by the string . Likewise, the string , interpreted as a character class expression, denotes the character class , the only difference being the order in which the characters are presented. By convention, literal character classes are returned in code point order.

If a character in a character class expression is preceded by a backslash, it is escaped, meaning that any special significance attached to it for character class expressions is suppressed (for that specific instance in the expression only).

The first and last characters in a character class expression can be open and close brackets ( and ), respectively. In this case, the expression is equivalent to the one obtained by removing the first and last characters. If either of the characters  and  appear anywhere but at the beginning and end, respectively, of the character class expression, they are taken literally. For example,  is equivalent to .