How can I use rejectedWith to perform an exact match of the error message?

I've recently picked up the JS unit test libraries Mocha, Chai and Chai-As-Promise. However, I'm running into a situation where I'm not sure if it's a default behaviour or it's something I've missed.

when I assert an error message rejected from a promise, it seems that as long as the expected error message is a substring of the actual error message, it assertion will pass. and below is an example:

var chai = require('chai');
var expect = chai.expect;
var chaiAsPromised = require('chai-as-promised');
chai.use(chaiAsPromised);

var q = require('q');

describe('demo test', function(){

    // a mock up promise function for demo purpose
    function test(input){
        var d = q.defer();

        setTimeout(function() {
            if(input){
                d.resolve('12345');
            }else{
                // throw a new Error after half a sec here
                d.reject(new Error('abcde fghij'));
            }
        }, (500));

        return d.promise;
    }

    // assertion starts here

    it('should pass if input is true', ()=>{
        return expect(test(true)).to.eventually.equal('12345');
    });

    it('this passes when matching the first half', ()=>{
        return expect(test(false)).to.be.rejectedWith('abcde');
    });

    it('this also passes when matching the second half', ()=>{
        return expect(test(false)).to.be.rejectedWith('fghij');
    });

    it('this again passes when even only matching the middle', ()=>{
        return expect(test(false)).to.be.rejectedWith('de fg');
    });

    it('this fails when the expected string is not a substring of the actual string', ()=>{
        return expect(test(false)).to.be.rejectedWith('abcdefghij');
    });
});

Is this the default behaviour? If it is, is there an option to force an exact match of the error message?

mocha@3.4.2 chai@4.0.2 chai-as-promised@7.1.1 q@1.5.0

Many Thanks.