I forgot to anchor. Should be

"\\A(?!.*[^\n])" or "\\A(?!.*\\S)"

Of course, the following are much simpler:

"\\A[^\n]*\\z" or "\\A\\s*\\z"

By the way, your test is wrong. You never actually attempt to match.

In reply to Re^3: Multiline match for empty string by ikegami
in thread Multiline match for empty string by josh803316

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