String replacement using Python 3 re.sub

Question

p = re.compile('Dhcp Server .*? add scope .*? (.*?) ')
test_str = 'Dhcp Server \\server1 add scope 10.0.1.0 255.255.255.0 "area1"'
subst = "255.255.254.0"
re.sub(p, subst, test_str)

The output is 255.255.254.0. What i am trying to get is this:

Dhcp Server \\server1 add scope 10.0.1.0 255.255.254.0 "area1"'

I can't simply use string replace because server1 and 10.0.1.0 will by dynamic.

How can I get my desired result when using Python 3.5? I looked at other SO questions, but did not find one quite like my question.

anubhava · Accepted Answer · 2015-10-06 15:53:52Z

2

You can use capturing groups for this:

test_str = 'Dhcp Server \\server1 add scope 10.0.1.0 255.255.255.0 "area1"'

print re.sub(r'(Dhcp Server .*? add scope [\d.]+) [\d.]+ (.*)', r"\1 255.255.254.0 \2", test_str)

We are capturing text before replacement position into \1 and part after replacement is available in \2.

Output:

Dhcp Server \\server1 add scope 10.0.1.0 255.255.254.0 "area1"

answered Oct 6, 2015 at 15:53

anubhava

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Comments

Barmar · Accepted Answer · 2015-10-06 16:01:39Z

1

You have it backwards. You use capture groups for the parts of the expression that you want to copy, not the parts you want to replace. Then you use a back-reference in the replacement string to copy them.

p = re.compile('(Dhcp Server .*? add scope .*? ).*? ')
test_str = 'Dhcp Server \\server1 add scope 10.0.1.0 255.255.255.0 "area1"'
subst = r"\g<1>255.255.254.0 "
re.sub(p, subst, test_str)

edited Oct 6, 2015 at 16:01

answered Oct 6, 2015 at 15:53

Barmar

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4 Comments

jftuga Over a year ago

This returns: 'U5.255.254.0 "area1"'

Barmar Over a year ago

Need to use \g<1> because it was looking for group number 125.

jftuga Over a year ago

Can you please explain what is meant by "group number 125"?

Barmar Over a year ago

When backslash is followed by a number, it means to substitute that capture group. My original answer had \1255.255.254.0 in the replacement, so it treated \125 as a back-reference to group number 125.

empy · Accepted Answer · 2015-10-06 16:25:00Z

1

Or you could just match the parts you want and then reconstruct the line (particularly if you want to know if you have made a replacement):

p = re.compile('(Dhcp Server .*? add scope .*? )(.*?)( .*)')
test_str = 'Dhcp Server \\server1 add scope 10.0.1.0 255.255.255.0 "area1"'
subst = "255.255.254.0"
match = p.match(test_str)
if match:
  replaced = match.group(1) + subst + match.group(3)

answered Oct 6, 2015 at 16:25

empy

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