Question

If I create a RegEx from an empty string, how do I make it match nothing?

I was looking for an option like RegexOptions.DoNotMatchWithEmpty or something like that.

To clarify:

Let’s assume I have no control over the string that is used to create the RegEx. The string can be empty and I have to create the RegEx object. I probably could set the RegEx to null and add a lot of null checking upstream but I’d rather not do that. Best solution would have been a RegexOptions flag that tells to match nothing when RegEx is constructed with an empty string. But that isn’t available.

Answer 1

How about just doing this

if (string.IsNullOrEmpty(regex))
{
    return false;
}
// standard regex lookup goes here

Update: Alternatively just return a regular expression that matches nothing

if (string.IsNullOrEmpty(regex))
{
    return new Regex("a^");
}
return new Regex(regex);

It works because the ^ character matches the start of the string being tested (unless the Multiline regex option is supplied, which it isn’t), however this clearly can’t match as we have already matched another character, a.