Python re.sub with a flag does not replace all occurrences
Solution 1:
Look at the definition of re.sub:
re.sub(pattern, repl, string[, count, flags])
The 4th argument is the count, you are using re.MULTILINE (which is 8) as the count, not as a flag.
Either use a named argument:
re.sub('^//', '', s, flags=re.MULTILINE)
Or compile the regex first:
re.sub(re.compile('^//', re.MULTILINE), '', s)
Solution 2:
re.sub('(?m)^//', '', s)