What is Regex or Regular expression?
Regex stands for Regular Expression and is a sequence of characters that specifies a search pattern, which can be utilized in the Exim filters to search for certain words or patterns within the message.
For more information regarding Regex, please read the following documentation: https://www.computerhope.com/jargon/r/regex.htm.
Please note that cPanel, LLC only supports the cPanel-provided software and does not provide general system-administration services. Therefore, cPanel Support cannot create a Regular Expression for you.