Manipulating stdin and redirect to stdout in Python

Question

I'm trying to write a simple python script where

1. it takes values from stdin
2. replaces a specific matched word
3. passes on the output with the NEW value back to stdout

I only have the part where it takes the values from stdin and looks for the matching words, I'm a bit stuck after that.

import re
import sys

for line in sys.stdin:
    matchObj = re.search(r'<something>(.*)</something>',line)
    if matchObj:
        oldWord = matchObj.group(1)
        print oldWord

Contents of foo

<something>REPLACEME</something>
<blah>UNTOUCH</blah>

Ideally if I run this command

cat foo | ./test.py

I would get something like this

<something>NEWWORD</something
<blah>UNTOUCH</blah>

Answer 1

Are you looking for re.sub ?

import re
import sys

for line in sys.stdin:
    sys.stdout.write(re.sub(r'(<something>)REPLACEME(</something>)',
                            r'\1NEWWORD\2',
                            line))

Running the above on your example data:

$ echo '<something>REPLACEME</something>\n<something>UNTOUCH</something>' | python2 test.py
<something>NEWWORD</something>
<blah>UNTOUCH</blah>

Note that parsing XML with regular expressions is probably a bad idea. The Python standard library comes with a number of XML modules .

Here's an example:

import sys
import xml.etree.ElementTree

tree = xml.etree.ElementTree.parse(sys.stdin)
root = tree.getroot()

for node in root.iter('something'):
    if node.text == 'REPLACEME':
        node.text == 'NEWWORD'

tree.write(sys.stdout)

The above would work just the same:

$ echo '<root><something>REPLACEME</something>\n<blah>UNTOUCH</blah></root>' | python2 test.py
<root><something>REPLACEME</something>
<blah>UNTOUCH</blah></root>

Answer 2

firs if you run cat foo | ./test.py cat foo | ./test.py you got test.py: command not found , you need to run this : cat foo |python ./test.py .

then the output of your code will be :

REPLACEME

but for the output that you want, you need to use re.sub() :

import re
import sys

for line in sys.stdin:
    matchObj = re.sub(r'<something>(.*)</something>','<something>NEWWORD</something>',line)
    if matchObj:
        print matchObj

output :

<something>NEWWORD</something>

<blah>UNTOUCH</blah>

Also as a pythonic way you can use The ElementTree XML API