regular expression for anything but an empty string

by 
^(?!\s*$).+

will match any string that contains at least one non-space character.

So

if (Regex.IsMatch(subjectString, @"^(?!\s*$).+")) {
    // Successful match
} else {
    // Match attempt failed
}

should do this for you.

^ anchors the search at the start of the string.

(?!\s*$), a so-called negative lookahead, asserts that it’s impossible to match only whitespace characters until the end of the string.

.+ will then actually do the match. It will match anything (except newline) up to the end of the string. If you want to allow newlines, you’ll have to set the RegexOptions.Singleline option.

Left over from the previous version of your question:

^\s*$

matches strings that contain only whitespace (or are empty).

The exact opposite:

^\S+$

matches only strings that consist of only non-whitespace characters, one character minimum.

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