Commonly they're enclosed between slashes (/like this/), because that's how they were used in many ancient Unix tools. They can also be used for text substitution, for example with the s/ construct. A common operator specifying that a RE will follow is the tilde (~).
The Perl language makes heavy use of regular expressions, and the grep tool also searches by REs. SED and AWK are some older tools also using REs.
REs come in two forms: Basic REs and Extended REs, varying slightly in syntax and possible contructs. Perl extens the Extended REs even further and has defined the standard "Perl Compatible REs" (PCRE).
result: 1 (because the regular expression between the // matches the string)
See regular expression for a proper use.
Today, the sun is shining, it is a sunny day today!
Butterfly is an insect.
True is not false.
Beautiful flowers have short lives.