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In formal language theory, the empty string, or empty word, is the unique string of length zero. Formally, a string is a finite, ordered sequence of characters such as letters, digits or spaces. The empty string is the special case where the sequence has length zero, so there are no symbols in the string. There is only one empty string, because two strings are only different if they have different lengths or a different sequence of symbols.
The empty string is the identity element of the concatenation operation. Reversal of the empty string produces the empty string. The empty string precedes any other string under lexicographical order, because it is the shortest of all strings. In context-free grammars, a production rule that allows a symbol to produce the empty string is known as an ε-production, and the symbol is said to be “nullable”.