在计算机科学中，字母表是字符或数字的有限集合。最常见的字母表是二元字母表{0,1}。有限字符串是来自字母表的字符的有限串行；例如二元字符串是来自字母表{0,1}的字符构成的字符串。字符的无限串行也可以用来自一个字母表的元素来构造。

给定一个字母表，我们写来指示在字母表上的所有有限字符串的集合。这里的指示Kleene星号算子。我们写(偶尔或）来指示在字母表上的所有无限串行的集合。

例如，如果我们使用二元字母表{0,1}，则字符串ε, 0, 1, 00, 01, 10, 11, 000,等都将在这个字母表的Kleene闭包中（这里的ε表示空串）。

In formal language theory, a string is defined as a finite sequence of members of an underlying base set; this set is called the alphabet of a string or collection of strings. The members of the set are called symbols, and are typically thought of as representing letters, characters, or digits. For example, a common alphabet is {0,1}, the binary alphabet, and a binary string is a string drawn from the alphabet {0,1}. An infinite sequence of letters may be constructed from elements of an alphabet as well.