I need to have regular expression which find a character which not following by same character after it. Mean exclude double even multiple same character.

For example, when i need to find `'e'` character from string: `"need only single characteeer"`, it is mean will find `'e'` on each words breakdown as below:

`"need"` > not match because it has double `'e'`

`"only"` > not match because no `'e'`

`"single"` > match because has only single `'e'`

`"characteeer"` > not match because has multiple of `'e'`

Not sure whether it is possible or not. Any answer or comment will be highly appreciated. Thanks in advance.

**UPDATE**

Maybe my question above still ambiguous. Actually i need to find the `'e'` character only instead the words. I am going to replace it with double character. So the one which already has double character will not replaced.

The main purpose is to replace `'e'` with `'ee'` for example. But the one which has `'ee'` or `'eee'` already, or even more `'e'`, will be untouched.