First you compile the regex, then you have to use it with match, find, or some other method to actually run it against some input.

import os
import re
import shutil

def test():
    os.chdir("C:/Users/David/Desktop/Test/MyFiles")
    files = os.listdir(".")
    os.mkdir("C:/Users/David/Desktop/Test/MyFiles2")
    pattern = re.compile(regex_txt, re.IGNORECASE)
    for x in (files):
        with open((x), 'r') as input_file:
            for line in input_file:
                if pattern.search(line):
                    shutil.copy(x, "C:/Users/David/Desktop/Test/MyFiles2")
                    break

The REPL makes it easy to learn APIs. Just run python, create an object and then ask for help:

$ python
>>> import re
>>> help(re.compile(r''))

at the command line shows, among other things:

search(...)

search(string[, pos[, endpos]]) –> match object or None.
Scan through string looking for a match, and return a corresponding
MatchObject instance. Return None if no position in the string matches.

so you can do

regex = re.compile(regex_txt, re.IGNORECASE)

match = regex.search(content)  # From your file reading code.
if match is not None:
  # use match

Incidentally,

regex_txt = "facebook.com"

has a . which matches any character, so re.compile("facebook.com").search("facebookkcom") is not None is true because . matches any character. Maybe

regex_txt = r"(?i)facebook.com"

The . matches a literal "." character instead of treating . as a special regular expression operator.

The r"..." bit means that the regular expression compiler gets the escape in . instead of the python parser interpreting it.

The (?i) makes the regex case-insensitive like re.IGNORECASE but self-contained.

Regex’s shouldn’t really be used in this fashion – unless you want something more complicated than what you’re trying to do – for instance, you could just normalise your content string and comparision string to be:

if 'facebook.com' in content.lower():
    shutil.copy(x, "C:/Users/David/Desktop/Test/MyFiles2")

if re.search(r'pattern', string):

Simple if-regex example:

if re.search(r'ingb', "seeking a great perhaps"):     # any words end with ing?
    print("yes")

Complex if-regex example (pattern check, extract a substring, case insensitive):

search_object = re.search(r'^OUGHT (.*) BE$', "ought to be", flags=re.IGNORECASE)
if search_object:
    assert "to" == search_object.group(1)     # what's between ought and be?

Notes:

• Use re.search() not re.match. The match method restricts to the start of the string, a confusing convention. If you want that, search explicitly with caret: re.search(r'^...', ...) (Or in re.MULTILINE mode use A)

• Use raw string syntax r'pattern' for the first parameter. Otherwise you would need to double up backslashes, as in re.search('ing\b', ...)

• In these examples, '\b' or r'b' is a special sequence meaning word-boundary for regex purposes. Not to be confused with 'b' or 'x08' backspace.

• re.search() returns None if it doesn’t find anything, which is always falsy.

• re.search() returns a Match object if it finds anything, which is always truthy.

• even though re.search() returns a Match object (type(search_object) is re.Match) I have taken to calling the return value a search_object. I keep returning to my own bloody answer here because I can’t remember whether to use match or search. It’s search, dammit.

• a group is what matched inside pattern parentheses.

• group numbering starts at 1.

• Specs

• Tutorial

With a running example:

"""
very nive interface to try regexes: https://regex101.com/
"""
# %%
"""Simple if statement with a regex"""
import re

regex = r"s*Proof.s*"
contents = ['Proof.n', 'nProof.n']
for content in contents:
    assert re.match(regex, content), f'Failed on {content=} with {regex=}'
    if re.match(regex, content):
        print(content)