Search type Search syntax
Tags [tag]
Exact "words here"
Author user:1234
user:me (yours)
Score score:3 (3+)
score:0 (none)
Answers answers:3 (3+)
answers:0 (none)
isaccepted:yes
hasaccepted:no
inquestion:1234
Views views:250
Sections title:apples
body:"apples oranges"
URL url:"*.example.com"
Favorites infavorites:mine
infavorites:1234
Status closed:yes
duplicate:no
migrated:no
wiki:no
Types is:question
is:answer
Exclude -[tag]
-apples
For more details on advanced search visit our help page
Results tagged with Search options answers only user 108088

Questions about regular expressions, a formalism to describe regular languages.

3
votes
$\Sigma$ does not include the empty string; it only contains the characters of the language, and the empty string is not a character. As user ttnick said in a comment, in general, if $\Sigma=\{x_1,x_ …
answered Jul 31 by thorimur