Matching pattern in Python

Question

I had a directory "/pcap_test" which contains several log files. Each file has a pattern like:

Pkt: 1 (358 bytes), LIFE: 1, App: itunes (INTO), State: TERMINATED, Stack: /ETH/IP/UDP/itunes, Error: None

Pkt: 2 (69 bytes), LIFE: 2, App: zynga (INTO), State: INSPECTING, Stack: /ETH/IP/UDP, Error: None

Pkt: 3 (149 bytes), LIFE: 2, App: pizzeria (INTO), State: TERMINATED, Stack: /ETH/IP/UDP/pizzeria, Error: None

In this case I want the output to be the second line because the content in the "App" is not present in the "Stack: "

I wrote a small Python script to iterate through the directory, open each file and print the output:

import os
list = os.listdir("/home/test/Downloads/pcap_test")
print list
for infile in list:
  infile = os.path.join("/home/test/Downloads/pcap_test" , infile)

if os.path.isfile(infile):
str = file(infile, 'r').read()
print str

I somehow got the output using grep but unable to use the same in the python script. Its something like:

grep -vP 'App: ([^, ]*) \(INTO\).*Stack: .*\1.*$' xyz.pcap.log | grep -P 'App: ([^, ]*) \(INTO\)'

Since I already have the file named "str" , I want to use that rather than individual log files, to get the output.

Any help in this regard will be highly appreciated.

Answer 1

First, I'd advise against variable names like str as that's Python's name for the String primitive data type.

Since grep is a command-line regular expression tool and since you already have a working regular expression all you need to do is learn to use Python's re module .

What's a little difficult is capturing grep's -v behaviour. I suggest reading the file line by line and printing the line only if it does not match your first regular expression but does match the second, like so:

if os.path.isfile(infile):
    with file(infile, 'r') as logFile: #this will close the file pointer automatically when you finish
        for line in logFile: #read logFile one line at a time
            firstReMatch = re.match(r'App: ([^, ]*) \(INTO\).*Stack: .*\1.*$', line) #check if this line matches your first regex
            secondReMatch = re.match(r'App: ([^, ]*) \(INTO\)', line) #check if this line matched your second regex
            if secondReMatch and not firstReMatch: #"not" to capture the inverse match
                print line #print the line.

Depending on your data, you may want to use re.search() instead of re.match()