validateemailaddress() ?

for validating email address you can use this regex 

regexmatch("^[\w\.\-]+@[\w\.\-]+\.[\w\.]{2,}$",ri!input)

This pattern fails a bunch of the test cases I have set up in my Validate Email Addresses expression rule.  Two of the most obvious/important failure cases is that it doesn't return "false" for email addresses beginning or ending with extraneous periods (".").

This is the pattern I use in mine, which passes a rather rigorous 18 test cases (though i can't guarantee it passes all realistic ones, just the ones i've been able to think of).

regexmatch(
  pattern: "^[A-Z0-9\'\+\&\%_-]+(\.{0,1}[A-Z0-9\'\+\&\%_-]+)*[@]{1}[A-Z0-9.-]*[A-Z0-9-]+[.]{1}[A-Z]{2,6}$",
  searchString: ri!email,
  regexFlags: "si"
)

Also note: all of the above test cases also correctly pass using the "validateEmailAddress()" function from People Functions, for what it's worth.

This pattern fails a bunch of the test cases I have set up in my Validate Email Addresses expression rule.  Two of the most obvious/important failure cases is that it doesn't return "false" for email addresses beginning or ending with extraneous periods (".").

This is the pattern I use in mine, which passes a rather rigorous 18 test cases (though i can't guarantee it passes all realistic ones, just the ones i've been able to think of).

regexmatch(
  pattern: "^[A-Z0-9\'\+\&\%_-]+(\.{0,1}[A-Z0-9\'\+\&\%_-]+)*[@]{1}[A-Z0-9.-]*[A-Z0-9-]+[.]{1}[A-Z]{2,6}$",
  searchString: ri!email,
  regexFlags: "si"
)

Also note: all of the above test cases also correctly pass using the "validateEmailAddress()" function from People Functions, for what it's worth.

Is there a way to display a message instead of a boolean value with this?

You would put the message at the parent (interface) level when this rule returns a FALSE value, for instance.

But really, it's an expression rule, you can make yours return any value you want - that's a pretty basic change.