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A regular expression, regex or regexp (sometimes called a rational expression) is, in theoretical computer science and formal language theory, a sequence of characters that define a search pattern. Usually this pattern is then used by string searching algorithms for "find" or "find and replace" operations on strings. The concept arose in the 1950s when the American mathematician Stephen Cole Kleene formalized the description of a regular language. Read more on Wikipedia...
Hello World contains a character other than a, b, and c.