如何检查字符串是否包含一个单词并且不包含另一个单词？

I have a string like this:

$str = "this is a test";

I want to validate $str and return true if string is containing is and it isn't containing test. How can I do that?

---

Examples:

"this is a test"   // false
"this is a tes"    // true    "is" exists and "test" doesn't exist
"this iss a tes"   // false
"this iss a test"  // false

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Here is my pattern \bis\b(?!test). But it seems to just checks existing, I mean it also returns true when test exists. I mean the result of following code us true which shouldn't be (because test exists).

if (preg_match ("/\bis\b(?!test)/","this is a test")) {
    return true;
} else {
    return false;
}

Note: I'm really insist on doing that by regex.