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Describes the combined set of all letters in the alphabet and the numbers 0 through 9. It is useful to group letters and numbers together because many programs treat them identically, and differently from punctuation characters. For example, most operating systems allow you to use any letters or numbers in filenames but prohibit many punctuation characters. Your computermanual would express this rule by stating: "Filenames may be composed of alphanumeric characters."

Sometimes additional characters are considered alphanumeric. For example, on IBM mainframesthe characters @, #, and $ are considered alphanumeric characters.

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