Bdtjtk

I have a large txt-file and want to extract all strings with these patterns:

/m/meet_the_crr

/m/commune

/m/hann_2

Here is what I tried:

import re



with open("testfile.txt", "r") as text_file:

    contents = text_file.read().replace("n", "")



print(re.match(r'^/m/[a-zA-Z0-9_-]+$', contents))

The result I get is a simple "None". What am I doing wrong here?

You need to not remove lineends and use the re.MULTILINE flag so you get multiple results from a bigger text returned:

# write a demo file

with open("t.txt","w") as f:

    f.write("""

/m/meet_the_crrn

/m/communen

/m/hann_2nn

# your text looks like this after .read().replace("\n","")n

/m/meet_the_crr/m/commune/m/hann_2""")

Program:

import re



regex = r"^/m/[a-zA-Z0-9_-]+$"



with open("t.txt","r") as f:

    contents = f.read()



found_all =  re.findall(regex,contents,re.M) 



print(found_all)

print("-")

print(open("t.txt").read())

Output:

['/m/meet_the_crr', '/m/commune', '/m/hann_2'] 

Filecontent:

/m/meet_the_crr



/m/commune



/m/hann_2





# your text looks like this after .read().replace("n","")



/m/meet_the_crr/m/commune/m/hann_2

This is about what Wiktor Stribiżew did tell you in his comment - although he suggested to use a better pattern as well: r'^/m/[w-]+$'

There is nothing logically wrong with your code, and in fact your pattern will match the inputs you describe:

result = re.match(r'^/m/[a-zA-Z0-9_-]+$', '/m/meet_the_crr')

if result:

    print(result.groups())    # this line is reached, as there is a match

Since you did not specify any capture groups, you will see () being printed to the console. You could capture the entire input, and then it would be available, e.g.

result = re.match(r'(^/m/[a-zA-Z0-9_-]+$)', '/m/meet_the_crr')

if result:

    print(result.groups(1)[0])



/m/meet_the_crr

You are reading a whole file into a variable (into memory) using .read(). With .replace("n", ""), you re,ove all newlines in the string. The re.match(r'^/m/[a-zA-Z0-9_-]+$', contents) tries to match the string that entirely matches the /m/[a-zA-Z0-9_-]+ pattern, and it is impossible after all the previous manipulations.

There are at least two ways out. Either remove .replace("n", "") (to prevent newline removal) and use re.findall(r'^/m/[w-]+$', contents, re.M) (re.M option will enable matching whole lines rather than the whole text), or read the file line by line and use your re.match version to check each line for a match, and if it matches add to the final list.

Example:

import re

with open("testfile.txt", "r") as text_file:

    contents = text_file.read()

    print(re.findall(r'^/m/[w-]+$', contents, re.M))

Or

import re

with open("testfile.txt", "r") as text_file:

    for line in text_file:

        if re.match(r'/m/[w-]+s*$', line):

            print(line.rstrip())

Note I used w to make the pattern somewhat shorter, but if you are working in Python 3 and only want to match ASCII letters and digits, use also re.ASCII option.

Also, / is not a special char in Python regex patterns, there is no need escaping it.