Python - Remove lines from output contains one or more substring from a list

Question

I want to remove lines from output that contains in them one of the substrings in my "pattern_list" with python re but keep the output as one string (without those lines).
So , I looked on re library and wrote the following code:

patterns_to_remove = ["$",":",">"] patterns = "|".join(patterns_to_remove) extra_lines_with_patterns = re.findall('\r\n.*{} \\w*'.format(re.escape(patterns)), str(output)) for extra_line in extra_lines_with_patterns: output = str(output).replace(extra_line, "") return output

So if my output is :

$a$ :b: ^c^

I want the output to be:

a b c

but I get always None in the end , I guess I did something wrong with the re flags.

You are not escaping the the pattern and pasting the alternations correctly, use re.findall('\r\n.*(?:{}) \\w*'.format("|".join(map(re.escape, patterns_to_remove))), str(output)) — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Aug 21, 2019 at 7:40
Actually, you need no findall, if you remove with regex, use re.sub — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Aug 21, 2019 at 7:46
Out of curiosity: Why don't you use a LC like print(''.join([x for x in output if x not in "$:^"])) ? — Ocaso Protal
– Ocaso Protal, Commented Aug 21, 2019 at 7:53

Wiktor Stribiżew · Accepted Answer · 2019-09-09 11:09:40Z

You escaped a part of a regex pattern with re.escape(patterns) and all | operators turned into literal pipes, \|. Also, you did not group them in the pattern when passing through format and the pattern looked like \r\n.* \$|\:|\> \w*, so it got corrupt (see Why do ^ and $ not work as expected?).

So you need to

Escape the patterns_to_remove with "|".join(map(re.escape, patterns_to_remove))
Enclose the {} with a (?:...), non-capturing group, i.e. '\r\n.*(?:{}) \\w*'

Use

re.findall('\r\n.*(?:{}) \\w*'.format("|".join(map(re.escape, patterns_to_remove))), str(output))

Or, since you are removing matches, just use re.sub:

patterns_to_remove = ["$",":",">"] output = re.sub('\r\n.*(?:{}) \\w*'.format("|".join(map(re.escape, patterns_to_remove))), '', str(output))

NOTE: '\r\n.*(?:{}) \\w*' = r'\r\n.*(?:{}) \w*'.

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