How to find out that a substring is not in the text?

Question

Regular Expressions

I want to find web pages that are not fully loaded by the program, for this you need to determine that the tag is not present in the text important! without using PHP or other languages, only regular expression </body>

Answer 1

(?s)^(?!.*</body>)

Answer 2

substring_is_not_in_text === NOT(substring_is_in_text)