def subn(pattern, repl, string, count=<NUM_LIT:0>, flags=<NUM_LIT:0>):
return _compile(pattern, flags).subn(repl, string, count)<EOL>
Return a 2-tuple containing (new_string, number).
    new_string is the string obtained by replacing the leftmost
    non-overlapping occurrences of the pattern in the source
    string by the replacement repl.  number is the number of
    substitutions that were made. repl can be either a string or a
    callable; if a string, backslash escapes in it are processed.
    If it is a callable, it's passed the match object and must
    return a replacement string to be used.