Regex - Remove everything from a string that does not match an expression

Question

I am creating a JavaScript function that will take a user inputted value and and create a CSS class name out of it. The following expression can be used to detect whether the end result follows valid css rules

-?[_a-zA-Z]+[_a-zA-Z0-9-]*

but I need to find a way to use it to remove all invalid characters.

I was thinking of something like:

var newClass = userInput.replace(EVERYTHING BUT /[_a-zA-Z0-9-]/, "");

Answer 1

A very small modification to your existing regex should work, using the ^ operator and g:

/[^a-zA-Z0-9_-]+/g

Which should be used as:

var newClass = userInput.replace(/[^a-zA-Z0-9-_]/g, '');

The ^ character, as the first character inside the brackets [], specifies to match what's not in the brackets (i.e. - the characters you want to strip).
The g modifier performs a global-match on the entire input string.