From LimeSurvey Manual
Validation (using regular expressions) (preg_validation)
This option lets you specify a regular expression with which you can validate all the parts of a question.
If any question or subquestion value does not meet the validation requirements, the background colour of the text entry field will be changed to red so that users can easily see which parts of the question need to be corrected. Thus, you can do on-page validation. In this way you will not need to wait for the user to click on the submit button to validate the respective question.
- Any valid regular expression
- Click here to check multiple examples.