Prevent multiline text in a single text field

In my form, I have a text question. I would like to prevent users from responding to this question witlh multiline responses. What is the constraint that i should use? From the documentation, there is this function:
contains(string, substring)

It was not my intention to accept multiple responses using this question but somehow the users discovered that they can input multiple responses by pressing the enter key and then typing in the next response. So I have instances where in this text question, there are 5 rows of responses, made possible by creating a new line using the Enter key.

I would like to know how to detect multiple rows using the contains function.


The parameter appearance set to multiline works in Enketo and appears to to enable use of the Enter key to add a new line. In Collect, multiline is not a valid option (as noted in the XLSForm reference table) - and pressing the button for a new line will expand the input box. I wonder if it would be possible and desirable to enforce something similar to the functionality in Enketo also in Collect?

I wonder if a regex expression can be used to test for new lines in text type question response in Collect?