flutter – Regex for ONLY positive and negative integer in input formatter for a TextFormField

I am trying to have a TextFormField which only accepts negative and positive number of 5 digits.

The following code but it does restrict digits only and up to 5, but it is not accepting a negative sign.

new TextFormField(
    maxLines: 1,
    keyboardType: TextInputType.number,
    enableSuggestions: false,
    inputFormatters: [
        LengthLimitingTextInputFormatter(5),
        FilteringTextInputFormatter.allow(RegExp(r'-?[0-9]')),
    ],
    onChanged: (val) {},
);

I have tried the following regex: “-?[0-9]” but it does not allow the negative sign.