I need to validate if the list is line delimited and in a valid email format using regex in paragraph field . If not , throw a validation message.test@test.com
test2@test.com
test3@test.com
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For validating an email address you can certainly contrive a complex regular expression to test for validity, though these days I just recommend using the "validateEmailAddress()" rule found in, i think, People Functions.
As far as "line delimited" - you'll need to implement your own parsing / checking for that part, depending on exactly what you want. It could be as simple as splitting the input on the linebreak character (char(10)) then validating each (non-empty) line in the resulting set - that would simply require looping over the validate email function.
I try to use the built-in Appian function for this as well. If that is not enough, I'll look at a regex solution.
I have an expression rule where I added ~18 extended test cases - edge cases to try to break the validation function - a while ago I developed a RegEx that handles all of them, and the plug-in rule (which came later iirc) handles all of them as well. I switch back and forth regarding which version to use - i haven't noticed any major performance difference between them (and they both use a plug-in ultimately so it's not like either version saves a dependency).