How to get accurate highlighting of modified lines in textfile

I've run into a big issue in my code.

Here's my code:

def highlight_nonmodified(content: str) -> str:
    regex = re.compile(r'(?s)(\{.*?[^\}]+\})', re.I | re.S)
    replace = r'#\1'
    content = regex.sub(replace, content)
    return content


def get_line(string_t: str, original: str) -> int:
    original = original.splitlines(True)
    for (i, line) in enumerate(original, 1):
        if string_t[1:] in line:
            return i

    return -1


def highligh_merge(original: str, modified: str) -> str:
    for line in modified.splitlines(True):
        if line.startswith('#'):
            numer = get_line(line, original)
            error = r"#Error: Tag not supported at line{0}\n".format(numer)
            error = error + line
            modified = modified.replace(line, error)

My issue is that here's what happens:

Textfile.txt (original):

Here goes some text. {tag} A wonderful day. It's soon cristmas. 
Happy 2019, soon. {Some useful tag!} Something else goes here. 
Happy ending. Yeppe! See you. 

Happy KKK! 
Happy B-Day!

Universe is cool!

{Tagish}. 

{Slugish}. Here goes another line. {Slugish} since this is a new sentence. 

endline.

Modified.txt:

Here goes some text.  A wonderful day. It's soon cristmas. 
Happy 2019, soon. #{Some useful tag!} Something else goes here. 
Happy ending. Yeppe! See you. 

Happy KKK! 
Happy B-Day!

Universe is cool!

. 

#Error: Tag not supported at line-1\n#{Slugish}. Here goes another line. #{Slugish} since this is a new sentence. 

endline. 

I cannot seem to get precise line numbering and comparing of lines, what am I doing wrong here, I am obviously, storing two copies, original and modified and then I pick then I try to pick out the line number from the original text by looping over line by line. But still without any success, is this even possible. Thanks a lot in advance!

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