Debugging invalid utf-8 characters in python3

Question

I am trying to debug why certain strings in my python3 script have non-utf8 characters. I found this script that should identify such characters:

https://rgxdb.com/r/5A2OX1FG

The web site gives python code for it:

regex = r"""
    (?:
        [\xC0-\xC1] # Invalid UTF-8 Bytes
      | [\xF5-\xFF] # Invalid UTF-8 Bytes
      | \xE0[\x80-\x9F] # Overlong encoding of prior code point
      | \xF0[\x80-\x8F] # Overlong encoding of prior code point
      | [\xC2-\xDF](?![\x80-\xBF]) # Invalid UTF-8 Sequence Start
      | [\xE0-\xEF](?![\x80-\xBF]{2}) # Invalid UTF-8 Sequence Start
      | [\xF0-\xF4](?![\x80-\xBF]{3}) # Invalid UTF-8 Sequence Start
      | (?<=[\x0-\x7F\xF5-\xFF])[\x80-\xBF] # Invalid UTF-8 Sequence Middle
      | (?<![\xC2-\xDF]|[\xE0-\xEF]|[\xE0-\xEF][\x80-\xBF]|[\xF0-\xF4]|[\xF0-\xF4][\x80-\xBF]|[\xF0-\xF4][\x80-\xBF]{2})[\x80-\xBF] # Overlong Sequence
      | (?<=[\xE0-\xEF])[\x80-\xBF](?![\x80-\xBF]) # Short 3 byte sequence
      | (?<=[\xF0-\xF4])[\x80-\xBF](?![\x80-\xBF]{2}) # Short 4 byte sequence
      | (?<=[\xF0-\xF4][\x80-\xBF])[\x80-\xBF](?![\x80-\xBF]) # Short 4 byte sequence (2)
    )
    """  

def stripNonUtf8(str):
    matches = re.search(regex, str, re.VERBOSE)
    if matches:
        print ("Match was found at {start}-{end}: {match}".format(start = matches.start(), end = matches.end(), match = matches.group()))

but I get the following error:

Traceback (most recent call last):
  File "log2db.py", line 330, in <module>
    main()
  File "log2db.py", line 325, in main
    stripNonUtf8("aaa")
  File "log2db.py", line 38, in stripNonUtf8
    matches = re.search(regex, str, re.VERBOSE)
  File "C:\ProgramData\Anaconda3\lib\re.py", line 183, in search
    return _compile(pattern, flags).search(string)
  File "C:\ProgramData\Anaconda3\lib\re.py", line 286, in _compile
    p = sre_compile.compile(pattern, flags)
  File "C:\ProgramData\Anaconda3\lib\sre_compile.py", line 764, in compile
    p = sre_parse.parse(p, flags)
  File "C:\ProgramData\Anaconda3\lib\sre_parse.py", line 930, in parse
    p = _parse_sub(source, pattern, flags & SRE_FLAG_VERBOSE, 0)
  File "C:\ProgramData\Anaconda3\lib\sre_parse.py", line 426, in _parse_sub
    not nested and not items))
  File "C:\ProgramData\Anaconda3\lib\sre_parse.py", line 816, in _parse
    p = _parse_sub(source, state, sub_verbose, nested + 1)
  File "C:\ProgramData\Anaconda3\lib\sre_parse.py", line 426, in _parse_sub
    not nested and not items))
  File "C:\ProgramData\Anaconda3\lib\sre_parse.py", line 736, in _parse
    p = _parse_sub(source, state, verbose, nested + 1)
  File "C:\ProgramData\Anaconda3\lib\sre_parse.py", line 426, in _parse_sub
    not nested and not items))
  File "C:\ProgramData\Anaconda3\lib\sre_parse.py", line 536, in _parse
    code1 = _class_escape(source, this)
  File "C:\ProgramData\Anaconda3\lib\sre_parse.py", line 309, in _class_escape
    raise source.error("incomplete escape %s" % escape, len(escape))
re.error: incomplete escape \x0 at position 411 (line 10, column 11)

what is going on?

Answer 1

Unlike in C, in Python it is required for a character with a hex value to be specified with exactly 2 digits.

Please refer to the documentation of String and Bytes literals , where it is noted:

Unlike in Standard C, exactly two hex digits are required.

So the code should be fixed with:

| (?<=[\x00-\x7F\xF5-\xFF])[\x80-\xBF] # Invalid UTF-8 Sequence Middle

In addition, the capabilities of Python's standard re module are relatively limited. You can install the regex module ( pip install regex ) and do import regex as re to work around the limitations.