There is this example of regular expressions:

$$(\Sigma\Sigma)^*= \{w\mid |w|\text{ is even}\}\,.$$

From that I understand the empty string is valid as a string of even length. Is this true?

  • 2
    $\begingroup$ Of course zero is even: it is two times zero. By the way: isnt there one $\Sigma$ too much in the expression? $\endgroup$ Feb 2, 2016 at 5:12
  • $\begingroup$ Ah ok I wasn't even thinking about zero. Nope that's the right number of $\Sigma$. $\endgroup$
    – Brian
    Feb 2, 2016 at 14:05

3 Answers 3


A number is even if it leaves no remainder when divided by two. Zero leaves no remainder when divided by two.


Even number - Even number = even number.
Odd number - odd number = even number.
0 - 0 = 0
Since it cannot change parity it must be even.

Furthermore 0's neighbours are odd, which also implies it is even (as every second number has the same parity).
Zero parity

  • 1
    $\begingroup$ What does advice on checking the parity of a number using bitmasks have to do with the question? $\endgroup$ Feb 2, 2016 at 8:12
  • $\begingroup$ Might as well explain the whole number theory. $\endgroup$
    – PleaseHelp
    Feb 2, 2016 at 8:40

Yes, that's correct. Empty string is valid as string of even length.

If you consider any language of even length of strings, it will also include an empty string.So, it is true.

  • $\begingroup$ "If you consider any language having even length of strings, it will also include an empty string." That's not true. For example, the language of all strings over $\{0,1\}$ that have even length and contain the string $000$ is a language of even-length strings that doesn't include $\epsilon$ (or any other string of length less than four). $\endgroup$ Feb 3, 2016 at 19:30
  • $\begingroup$ @DavidRicherby I meant this ' All the languages of even length strings(but don't contain strings other than even length strings ) include empty strings too.' Edit has been done. :) $\endgroup$
    – Parth S.
    Feb 4, 2016 at 12:49
  • $\begingroup$ But it's not true that all languages of even length strings include the empty string! I just gave you an example showing that that's false. $\endgroup$ Feb 4, 2016 at 15:34
  • $\begingroup$ @DavidRicherby so can we say that the language of all even length strings including null contains empty string? $\endgroup$
    – Parth S.
    Feb 4, 2016 at 21:55
  • $\begingroup$ Yes, absolutely. $\endgroup$ Feb 4, 2016 at 22:21

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