General :: Replaces Empty Spaces By Underscore?

my requirement is i need to copy mail subject to a text file and create one directory from the name of subject.

my MAIL subject is like this :

Quote:

Subject:Thanks to linuxquestions.org

i copied the mail subject to a file. and now the issue is to creating a directory.

now i want to create a directory without empty spaces, insted of space replace _ in that place.

i want out put like this.

Quote:

Thanks_to_linuxquestions.org