How can I invert a regular expression in JavaScript?

javascript regex invert
acme picture acme · Oct 8, 2009 · Viewed 56.6k times · Source

I have a string A and want to test if another string B is not part of it. This is a very simple regex whose result can be inverted afterwards.

I could do:

/foobar/.test('[email protected]')

and invert it afterwards, like this:

!(/foobar/).test('[email protected]')

The problem I have is, that I need to do it within the regular expression and not with their result. Something like:

/!foobar/.test('[email protected]')

(which does not work)

In other words: the regular expression should test for a non-existence and return true in that case.

Is this possible with JavaScript?

Answer

Bart Kiers picture Bart Kiers · Oct 8, 2009

Try:

/^(?!.*foobar)/.test('[email protected]')

A (short) explanation:

^          # start of the string 
(?!        # start negative look-ahead
  .*       # zero or more characters of any kind (except line terminators)
  foobar   # foobar
)          # end negative look-ahead

So, in plain English, that regex will look from the start of the string if the string 'foobar' can be "seen". If it can be "seen" there is no* match.

* no match because it's negative look-ahead!

More about this look-ahead stuff: http://www.regular-expressions.info/lookaround.html But Note that JavaScript only supports look-aheads, no look-behinds!

Related questions

How to validate an email address in JavaScript

Is there a regular expression to validate an email address in JavaScript?

Read More
Check whether a string matches a regex in JS

I want to use JavaScript (can be with jQuery) to do some client-side validation to check whether a string matches the regex: ^([a-z0-9]{5,})$ Ideally it would be an expression that returned true or false. I'm a JavaScript newbie, does …

Read More
How do you use a variable in a regular expression?

I would like to create a String.replaceAll() method in JavaScript and I'm thinking that using a regex would be most terse way to do it. However, I can't figure out how to pass a variable in to a regex. …

Read More