Empty string

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.

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.

These properties may hold in some programming languages, but this is left up to the particular implementation.