Regex For Special Character (S with line on top)

I was trying to write regex in Python to replace all non-ascii with an underscore, but if one of the characters is “S̄” (an ‘S’ with a line on the top), it adds an extra ‘S’… Is there a way to account for this character as well? I believe it’s a valid utf-8 character, but not ascii

Here’s there code:

import re
line = "ra*ndom wordS̄"
print(re.sub('[W]', '_', line))

I would expect it to output:

ra_ndom_word_

But instead I get:

ra_ndom_wordS__

Answer

The reason Python works this way is that you are actually looking at two distinct characters; there’s an S and then it’s followed by a combining macron U+0304

In the general case, if you want to replace a sequence of combining characters and the base character with an underscore, try e.g.

import unicodedata

def cleanup(line):
    cleaned = []
    strip = False
    for char in line:
        if unicodedata.combining(char):
            strip = True
            continue
        if strip:
            cleaned.pop()
            strip = False
        if unicodedata.category(char) not in ("Ll", "Lu"):
            char = "_"
        cleaned.append(char)
    return ''.join(cleaned)

By the by, W does not need square brackets around it; it’s already a regex character class.

Python’s re module lacks support for important Unicode properties, though if you really want to use specifically a regex for this, the third-party regex library has proper support for Unicode categories.

"Ll" is lowercase alphabetics and "Lu" are uppercase. There are other Unicode L categories so maybe tweak this to suit your requirements (unicodedata.category(char).startswith("L") maybe?); see also https://www.fileformat.info/info/unicode/category/index.htm