Regular expressions in Selenium IDE can be used with the keyword – regexp: as a prefix to the value and patterns needs to be included for the expected values.
For example if you want to use the regular expression for a command
Command: verifyText
Target: //font/font/b/font[1]
Value: Flight Confirmation # 2011-05-02451
in the above example Flight Confirmation is continuously changing each time you run the test case. So this can be written with a regular expression as mentioned below
Command: verifyText
Target: //font/font/b/font[1]
Value: regexp:Flight Confirmation # [0-9]{4}-[0-9]{2}-[0-9]{5,10}.