Bukkit

After you have selected a sign for use (using the default config):

_

/editSign hi_this_is \n a_sign

outputs:

hi this is

[NOTHING HERE]

a sign

_

/editsignln 2 a really cool

outputs:

hi this is

a really cool

a sign

_

/replaceln 3 a

outputs:

hi this is

a really cool

sign

_

/replacelnAll 1 i o

outputs:

ho thos os

a really cool

sign

_

/replacelnAll 1 \040

outputs:

hothosos

a really cool

sign

To see why that works click here and learn more about regular expressions.