In the DNS Zone Manager interface, the regexp field for NAPTR records does not allow an "" input or an empty field.

Symptoms

When attempting to add a NAPTR record, the regexp field cannot use "" or be left empty as it will error with the following:

To separate regular and replacement expressions, you must enter the delimiter before, between, and after the expressions. For example, delim-char regex delim-char replacement delim-char.

Or 

Error: API failure: (XID kn5ea5) “domain.com. 14400 IN NAPTR 10 100 "S" "SIP+D2U" _sip._udp.domain.com.” does not form a valid DNS resource record. (Syntax error, value expected)

Description

This is due to the field only accepting regexp strings that meet the above delimiter requirements and not accepting empty values.

We've opened an internal case for our development team to investigate this further. For reference, the case number is CPANEL-52754. Follow this article to receive an email notification when a solution is published in the product. 

Workaround

If using the S flag, the regexp field is ignored and any string meeting the requirement can be placed such as:

!.!!