Empty string

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In computer science and formal language theory, the empty string is the unique string of length zero. It is denoted with "λ" or sometimes ϵ.

The empty string is distinct from a null reference in that in an object-oriented programming language a null reference to a string type doesn't point to a string object and will cause an error were one to try to perform any operation on it. The empty string is still a string upon which string operations may be attempted.

[edit] Properties

When present in a formal language, empty strings have several properties:

$| \lambda \, | = 0$ . The string length is zero.
$\lambda \, + s = s + \lambda \, = s$ . Under concatenation, the empty string is the identity element of the free monoid on the alphabet Σ.
${ \lambda \, }^R = \lambda \,$ . Reversal of the empty string produces the empty string.