#download/try this: http://ideone.com/QA6Fg

from re import *
 
s = 'a->b\na->b\nc->b\nc->b\na->d\na->d\ne->b\ne->b\na->f\na->f\ng->b\ng->b\na->h\na->h\ni->b\ni->b\na->j\na->j\nk->b\nk->b\n'
res = subn(r"(.*\n)(\1)+",r"replaced:\1",s,M)
 
print("output: " + res[0])
print("replace count: %d" % res[1])
 
# if it works right the next print wont show
if res[1] == 8: print("WEIRD: too few replacements ("+str(res[1])+")")

#i'd expect this to replace all ocurrences not only the first 8 times

Argument 4 of re.subn(...) is 'count', the maximum number of replacements to perform, but you're passing in the MULTILINE flag, which happens to have the integer value 8, hence you're limiting the maximum number of replacements to 8.

aww ic