What does Regexextract return if no string is found

I am trying to make a custom filter where the I need to filter where a Sequence needs to match on


Basically examples like "-23", "-49", and "-35". These arent negative numbers, they are part of a string. I must also exclude 5 in the first digit after the hypen. I am using the Regexextract function, but not finding good information on what it returns if no match is found.