The empty regular expression
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This blog post takes a look at the empty regular expression.
The empty regular expression
An empty regular expression matches everything.
> var empty = new RegExp("");
> empty.test("abc")
true
> empty.test("")
true
As you probably know, you should only use the RegExp constructor when you are dynamically creating a regular expression. But how do you create it via a literal, given that you can’t use // (the token that starts a line comment)? This is how:
var empty = /(?:)/;
(?:) is an empty non-capturing group. Such a group leaves few traces and thus is a good choice. Even JavaScript itself uses the above representation when displaying an empty regular expression:
> new RegExp("")
/(?:)/
RegExp.prototype
Interestingly, the prototype object of regular expressions is also a regular expression, the empty regular expression:
> RegExp.prototype
/(?:)/
You can use RegExp.prototype like any other regular expression:
> "abc".match(RegExp.prototype)
[ '', index: 0, input: 'abc' ]
The regular expression that matches nothing
The empty regular expression has an inverse – the regular expression that matches nothing:
> var never = /.^/;
> never.test("abc")
false
> never.test("")
false
