Include? characters javascript validation

I am trying to write my own javascript validation and am looking for
some help with writing a function that checks to see if a string
contains invalid characters. I have gotten it to work for spaces:

username.include?(' ')

but I would like to perform a thorough check for all invalid
characters like my user validation:

validates_format_of :username, :with => /^[A-Z0-9_]*$/i

Im a little confused about the /^[A-Z0-9_]*$/i syntax. Can anyone
enlighten me?

/^[A-Z0-9_]*$/i is a regular expression.


^ denotes starts with
[A-Z0-9_]* denotes 0 or more capitals, numbers and underscores
$ denotes ends_with

The i at the end means to ignore case (i think).

Thus your username can only contain letters, numbers and underscores
and nothing else.

The regular expression is contained between the //

If you were wanting to do this in a javascript function you will have
to investigate using regular expressions in javascript.
The syntax should be the same for the actual regular expression.