Regular expressions are used in programming languages to match parts of strings. You create patterns to help you do that matching.
If you want to find the word <code>"the"</code> in the string <code>"The dog chased the cat"</code>, you could use the following regular expression: <code>/the/</code>. Notice that quote marks are not required within the regular expression.
JavaScript has multiple ways to use regexes. One way to test a regex is using the <code>.test()</code> method. The <code>.test()</code> method takes the regex, applies it to a string (which is placed inside the parentheses), and returns <code>true</code> or <code>false</code> if your pattern finds something or not.