[exim] How to spell a regexp with a space in an Exim filter?

Three possibilities, I don't know which one would work:

$h_x-foo: matches \N"foo[.]? bar$"\N

$h_x-foo: matches "\Nfoo[.]? bar$\N"

$h_x-foo: matches \Nfoo[.]? bar$\N

The documentation, alas, doesn't help.

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