RegExps, shorthand for regular expressions, are used in pattern matching and substitution operators.
Regular expressions are actually a grammer for a little language.
The Engine then returns a Boolean value, which depends on whether or not the string can be parsed as a sentence of your little language. A regular expression is really just a sequence or a pattern of characters that is matched against a string of text when performing searches and replacements. A simple regular expression consists of a single character or a set of characters that matches itself. Regexps are a very powerful tool. They pack a lot of meaning into a short space.