In PowerShell, regular expressions can be used to match patterns in strings using the -match operator. To write a regular expression in PowerShell, you can use the following syntax: '/pattern/'. For example, to check if a string contains the word "hello", you can write the following regular expression: '/hello/'. This will return true if the string contains the word "hello" and false otherwise. You can also use regular expression quantifiers, such as *, +, and .