You want to apply your own validation to a question. For instance, a code which starts with the same three digits as default. Or an answer must contain a capital letter between A to Z. This complex validation can be set with a regular expression. In Spotler forms, a regular expression can be set at the settings of a question. Double click on the question and click on Settings. You see Specific validation. Here you can set the regular expression.
Want to learn more about regular expression? Read all about it on: www.vanheusden.com/misc/regexp.html