Regex.IsMatch always returning true?

The following code was failing:

[TestMethod]
public void ShouldNotMatchInvalidCharacters()
{
    Regex regex = null;
    string invalidString = null;
    bool result = false;

    Story.WithScenario("matching invalid characters")
        .Given("a regex expression",
            () => regex = new Regex(@"[0-9]*\.{0,1}[0-9]*"))
        .And("an invalid string",
            ()=> invalidString = "a")
        .When("we check whether we have a match",
            () => result = regex.IsMatch(invalidString))
        .Then("the match should fail",
            () => Assert.IsFalse(result));
}

It turns out it was matching on the empty string. I couldn’t think why on earth it was doing this. A bit of googling and the correct way to match on an entire string is to specify the start and end of the string in the regex.

^[0-9]*\.{0,1}[0-9]*$

The test now goes green!