What are regular expressions in Cucumber?

A regular expression is a pattern describing a certain amount of text.  The most basic regular expression consists of a single literal character.

We use regular expression in Cucumber to write Step Definitions.

Step Definition in Java using Regular Expression

@When("^I fill in \"([^\"]*)\" with:$")
public void i_fill_in_with(String elementName, String messageText) 
{
	driver.findElement(By.id("message")).sendKeys(messageText);
}