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("^f oo", "\nfoo", re.MULTILINE)

<_sre.SRE_Mat ch object at 0x6c448>

This doesn't. No replacement:

>>re.sub("^foo" , "bar", "\nfoo", re.MULTILINE)

'\nfoo'

Why?

Thanks,
nyenyec