What Is Kleene Star?

In the parlance of computer science, a Kleene star is a programming resource that offers outcomes related to the concatenation of a string set. It can assess how to filter given results based on input. In other words, it's like this: You're trying to get some good deals on products on sale at your favorite store before they're gone forever (or until another sale rolls around). You've found a few items you want to buy, but you're concerned that they may not be there when you return. So, instead of just buying them all and hoping for the best, you think about what would happen if another customer came in with an item from each category of sale items—what then? Then you could use a Kleene star: if all three items are available in stock simultaneously, buy them! Otherwise, don't waste your time or money. The Kleene star, a less-known cousin of the asterisk, is used to indicate that a string can be repeated any number of times. Let's look at an example: If the input string is composed of the word "cat," the set of resulting Kleene star strings will include the results "cat" and "ccat" as well as "ccatt," "cccat," and "ccaattt." This may seem like many repetitions for a single word, but if you think about it this way: if you were to operate on each letter in this string (e.g., make all vowels uppercase), you will get a different result than if you performed it on only one letter at a time (e.g., make just one vowel uppercase). Imagine you're an engineer or professional in the field of automation. You've got a big data set in your IT system, and you need to decide which strings are valid inputs. Your first thought might be to use the Kleene star (aka "regular expression"). The Kleene star is a way of matching sets of strings, so it can tell you whether or not one string is part of another set of strings. But what if you have a massive set? What if your data comes from all over the world and includes every possible string that could be typed into your system? How do you know if any string is valid? The answer: You don't! You have to guess!