So how does such a regular expression pattern look like? Well, it can be very simple like [AB], which will match the letter A or B. There are also special characters which indicate how many times a letter can appear in the text, e.g. [AB]* which means that A or B can occur zero or more times. The syntax can get a little more advanced than this, but you can always find a quick reference by clicking " Help" in the top right menu above.