What is a regex to match ONLY an empty string?

Question

There are lots of posts about regexs to match a potentially empty string, but I couldn't readily find any which provided a regex which only matched an empty string.

I know that ^ will match the beginning of any line and $ will match the end of any line as well as the end of the string. As such, /^$/ matches far more than the empty string such as " ", "foobar ", etc.

I would have thought, though, that /A/ would match just the empty string, since A matches the beginning of the string and matches the end of the string. However, my testing shows that /A/ will also match " ". Why is that?

Answer 1

I would use a negative lookahead for any character:

^(?![sS])

This can only match if the input is totally empty, because the character class will match any character, including any of the various newline characters.