[Sed] Parse s/foo/bar strings in a simpler way

Instead of using a large, complicated regex, we use re.split() to split on
"a / which is not preceeded by a \" then count the resulting parts to determine
if it was correctly formatted.

This logic was split out into a seperate function.
Merge pull request #44 from ekimekim/master

[Sed] Parse s/foo/bar strings in a simpler way