Hello,
As far as we know, it’s specified as returning a boolean: https://getodk.github.io/xforms-spec/#fn:regex
Also pay attention: There is an essential difference in regex behaviour between Collect and Enketo. See previous discussion in community. https://community.kobotoolbox.org/t/regex-problem-in-kobocollect/7140/18. See also support article (at the bottom): https://support.kobotoolbox.org/restrict_responses.html?highlight=regex
