Python - Extract Code from Text using Regex

Question

I am a Python beginner and looking for help with an extraction problem.

I have a bunch of textfiles and need to extract all special combinations of an expression ("C"+"exactly 9 numerical digits") and write them to a file including the filename of the textfile. Each occurence of the expression I want to catch start at the beginning of a new line and ends with a "/n".

sample_text = """Some random text here 
and here
and here
C123456789
some random text here
C987654321
and here
and here"""

What the output should look like (in the output file)

My_desired_output_file = "filename,C123456789,C987654321"

My code so far:

min_file_size = 5

def list_textfiles(directory, min_file_size): # Creates a list of all files stored in DIRECTORY ending on '.txt'
    textfiles = []
    for root, dirs, files in os.walk(directory):
        for name in files:
            filename = os.path.join(root, name)
            if os.stat(filename).st_size > min_file_size:
                textfiles.append(filename)

for filename in list_textfiles(temp_directory, min_file_size):         
    string = str(filename)
    text = infile.read()
    regex = ???
    with open(filename, 'w', encoding="utf-8") as outfile:
       outfile.write(regex)

Answer 1

your regex is '^C[0-9]{9}$'

^           start of line
C           exact match
[0-9]       any digit
{9}         9 times
$           end of line

Answer 2

import re

regex = re.compile('(^C\d{9})')
matches = []
with open('file.txt', 'r') as file:
    for line in file:
        line = line.strip()
        if regex.match(line):
            matches.append(line)

You can then write this list to a file as needed.

Answer 3

How about:

import re

sample_text = """Some random text here 
and here
and here
C123456789
some random text here
C987654321
and here
and here"""

k = re.findall('(C\d{9})',sample_text)

print(k)

This will return all occurrences of that pattern. If you yield line from your text and store your target combination. Something like:

Updated:

import glob
import os
import re

search = {}
os.chdir('/FolderWithTxTs')
for file in glob.glob("*.txt"):
    with open(file,'r') as f:
        data = [re.findall('(C\d{9})',i) for i in f]
        search.update({f.name:data})

print(search)

This would return a dictionary with file names as keys and a list of found matches.