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.

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.

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.

Representations

Programming language	λ representation
C, C++	"" {'\0'}
C++	std::string()
Perl	"" '' qw()
Python	"" '' str()
C#	"" string.Empty
PHP	"" ''
Java	""
Haskell	""
Delphi	''

See also

• Empty set
• Null