CODEWITHSUNDEEP
pythonregex
Maxmoe
Maxmoe

Reputation: 101

Regular Expression in python doesn't work

I'm working on the exercise in the book Python for Informatics which asks me to write a program to simulate the operation of the grep command on UNIX. However, my code doesn't work. Here I simplified my code and only intend to calculate how many lines start with the word 'Find'. I'm quite confused and wish you could cast light on it.

from urllib.request import urlopen
import re

fhand = urlopen('http://www.py4inf.com/code/mbox-short.txt')
sumFind = 0

for line in fhand:
    line = str(line) #convert from byte to string for re operation
    if re.search('^From',line) is not None:
        sumFind+=1

print(f'There are {sumFind} lines that match.')

The output of the script is

There are 0 lines that match.

And here is the link of the input text: text

Thanks a lot for your time.

Upvotes: 1

Views: 431

Answers (2)

Tom
Tom

Reputation: 1133

You're issue is that the urllib module returns bytes instead of strings from the url/text file.

You can either:

  1. Use bytes in your regex search: re.search(b'From', line).

  2. Use requests module to download file as string and split by lines:

    import requests

    txt = requests.get('http://www.py4inf.com/code/mbox-short.txt').text.split('\n')

    for line in txt: ...

Upvotes: 0

Jean-François Fabre
Jean-François Fabre

Reputation: 140188

the mistake is to convert bytes to string using str.

>>> str(b'foo')
"b'foo'"

You would have needed

line = line.decode()

But the best way is to pass a bytes regex to the regex, that is supported:

for line in fhand:
    if re.search(b'^From',line) is not None:
        sumFind+=1

now I get 54 matches.

note that you could simplify the whole loop to:

sum_find = sum(bool(re.match(b'From',line)) for line in fhand)
  • re.match replaces the need to use ^ with search
  • no need for loop, sum counts the times where re.match returns a truthy value (explicitly converted to bool so it can sum 0 or 1)

or even simpler without regex:

sum_find = sum(line.startswith(b"From") for line in fhand)

Upvotes: 6

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