"\n" matches both '[\n]' and '[^\n]'

[ZyX]

While trying to purge comment lines inside a variable, I encountered the 
following problem:

    :echo substitute("a\nb", '[\n]', '<NL>', 'g')
    a<NL>b
    :echo substitute("a\nb", '[^\n]', '<!NL>', 'g')
    <!NL><!NL><!NL>

app-editors/vim-7.2.303 (Gentoo amd64),
not fixed in
app-editors/vim-7.2.442 (Gentoo ~amd64)

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