re.sub and re.MULTILINE

[nyenyec]

I feel like a complete idiot but I can't figure out why re.sub won't
match multiline strings:

This works:
>>> re.search("^foo", "\nfoo", re.MULTILINE)
<_sre.SRE_Match object at 0x6c448>

This doesn't. No replacement:
>>> re.sub("^foo", "bar", "\nfoo", re.MULTILINE)
'\nfoo'

Why?

Thanks,
nyenyec