Question type - Short free text/49/en
From LimeSurvey Manual
Sub-question validation equation (em_validation_sq)
This is an equation that is used to validate each subquestion (text field) individually. Any text field that does not pass these validation conditions will have its background color turned pink (using the .em_sq_fn_validation CSS style) to highlight the error. Note that this is available in addition to the regular expression-based validation option.
Any equation that makes use of the ExpressionScript syntax, without surrounding curly braces.
For example, if you want to allow only numbers that are a multiple of 3 as answers, the equation would be:
(this / 3) == floor(this/3)
The reserved variable this is automatically replaced by a reference to the active text entry cell.