The term, language , when used in the sense borrowed from automata theory, can be a bit confusing. For the regular expression to have meaning, this set of possible strings that we are defining should have some meaning to us. Regular expressions give us extreme power to do pattern matching on text documents. We can use the regular expression syntax to write a succinct description of the entire, infinite class of strings that fit our specification. In addition, anyone else who understands the description language of regular expressions, can easily read out description and determine what set of strings we want to match.
Regular expressions are a universal description for matching regular strings.
If it does not match, then the string is not in the desired class. All the other regular expression operators can actually be reduced into a set of these simple operators. In regular expressions, generally, a character matches itself. The only exceptions are regular expression special characters. For example, the regular expression abc matches a set of strings that contain abc somewhere in them.