r = re.compile(r"^s+", re.MULTILINE)
r.sub("", "a
b
c") # "a
b
c"
# or without compiling (only possible for Python 2.7+ because the flags option
# didn't exist in earlier versions of re.sub)
re.sub(r"^s+", "", "a
b
c", flags = re.MULTILINE)
# but mind that s includes newlines:
r.sub("", "a
b
c") # "a
b
c"