A Regular Expression is a sequence of characters that restricts a text field's input by a participant to a specified text format.
Examples of Regular Expressions include:
Phone number: ^[\+]?[(]?[0-9]{3}[)]?[-\s\.]?[0-9]{3}[-\s\.]?[0-9]{4,6}$
Email: [^@ \t\r\n]+@[^@ \t\r\n]+\.[^@ \t\r\n]+
Postcode: ([Gg][Ii][0[Aa]{2})|((([A-Za-z][0-9]{1,2})|(([A-Za-z][A-Ha-hJ-Yj-y][0-9]{1,2})|(([A-Za-z][0-9][A-Za-z])|([A-Za-z][A-Ha-hJ-Yj-y][0-9][A-Za-z]?))))\s?[0-9][A-Za-z]{2})
Password: ^(?=.*?[A-Z])(?=.*?[a-z])(?=.*?[0-9])(?=.*?[#?!@$ %^&*-]).{8,}$
IP address: (\b25[0-5]|\b2[0-4][0-9]|\b[01]?[0-9][0-9]?)(\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)){3}
Credit Card number: (^4[0-9]{12}(?:[0-9]{3})?$)|(^(?:5[1-5][0-9]{2}|222[1-9]|22[3-9][0-9]|2[3-6][0-9]{2}|27[01][0-9]|2720)[0-9]{12}$)|(3[47][0-9]{13})|(^3(?:0[0-5]|[68][0-9])[0-9]{11}$)|(^6(?:011|5[0-9]{2})[0-9]{12}$)|(^(?:2131|1800|35\d{3})\d{11}$)
Regular Expressions can be selected as a field value on the edit page. The Expression is added on the first line, and the error message is added as the second line. This will be a popup if the participant inputs text not in the specified format.
Here is an example of a signing page where the error message has popped up.