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Argument 4 of re.sub is the maximum number of replacements, NOT flags:
Help on function sub in module re:
sub(pattern, repl, string, count=0, flags=0)
 Return the string obtained by replacing the leftmost
 non-overlapping occurrences of the pattern in string by the
 replacement repl. 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.
re.I is 2, so you're telling it to perform at most 2 replacements.