Empty string

In formal language theory, the empty string 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. In formal treatments, the empty string is denoted with ε or sometimes Λ or λ.

The empty string should not be confused with the empty language ∅, which is a formal language (i.e. a set of strings) that contains no strings, not even the empty string.

The empty string has several properties:

In most programming languages, strings are a data type. Individual strings are typically stored in consecutive memory locations. This means that the same string (for example, the empty string) could be stored in two different places in memory. (Note that even a string of length zero can require memory to store it, depending on the format being used.) In this way there could be multiple empty strings in memory, in contrast with the formal theory definition, for which there is only one possible empty string. However, a string comparison function would indicate that all of these empty strings are equal to each other.

In most programming languages, the empty string is distinct from a null reference (or null pointer) because a null reference does not point to any string at all, not even the empty string. The empty string is a legitimate string, upon which most string operations should work. Some languages treat some or all of the following in similar ways, which can lessen the danger: empty strings, null references, the integer 0, the floating point number 0, the boolean value false, the ASCII character NUL, or other such values.

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