Python string.replace regular expression

Question

I have a parameter file of the form:

parameter-name parameter-value

Where the parameters may be in any order but there is only one parameter per line. I want to replace one parameter's parameter-value with a new value.

I am using a line replace function posted previously to replace the line which uses Python's string.replace(pattern, sub). The regular expression that I'm using works for instance in vim but doesn't appear to work in string.replace().

Here is the regular expression that I'm using:

line.replace("^.*interfaceOpDataFile.*$/i", "interfaceOpDataFile %s" % (fileIn))

Where "interfaceOpDataFile" is the parameter name that I'm replacing (/i for case-insensitive) and the new parameter value is the contents of the fileIn variable.

Is there a way to get Python to recognize this regular expression or else is there another way to accomplish this task?

score 713 · Accepted Answer · edited Nov 17 '17 at 00:06

713

str.replace() ^v2|v3 does not recognize regular expressions.

To perform a substitution using a regular expression, use re.sub() ^v2|v3.

For example:

import re

line = re.sub(
           r"(?i)^.*interfaceOpDataFile.*$", 
           "interfaceOpDataFile %s" % fileIn, 
           line
       )

In a loop, it would be better to compile the regular expression first:

import re

regex = re.compile(r"^.*interfaceOpDataFile.*$", re.IGNORECASE)
for line in some_file:
    line = regex.sub("interfaceOpDataFile %s" % fileIn, line)
    # do something with the updated line

edited Nov 17 '17 at 00:06

Alan W. Smith

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answered May 23 '13 at 17:53

Andrew Clark

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I had to pass in `flags=re.MULTILINE` as the last argument to `re.sub` in order to get this to work, which makes sense - [read about it in the docs here](https://docs.python.org/2/library/re.html#re.MULTILINE) – tobek Mar 11 '16 at 23:26
21

regex compilations are cached ([docs](https://docs.python.org/3.6/library/re.html#re.compile)), so compiling isn't even necessary. But as you show, if one compiles, compile outside the loop. – alttag Nov 14 '17 at 22:01
1

For anyone that wants to know the details of whether or not to compile, check [this answer](https://stackoverflow.com/a/452143). – Lual Feb 15 '20 at 21:59

score 505 · Answer 2 · edited Aug 08 '22 at 05:13

505

You are looking for the re.sub function.

import re
s = "Example String"
replaced = re.sub('[ES]', 'a', s)
print(replaced)

will print axample atring

edited Aug 08 '22 at 05:13

Hugo

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answered May 23 '13 at 17:55

Jacek Przemieniecki

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Is it possible to replace with case in one line, like if first character is E change to G, if Z change it to B for example. – GurhanCagin Dec 17 '18 at 12:50
You can pass a string -> string function as the second argument of `re.sub`. That function will be called with each matched substring, and its result will be put in the result. – Jacek Przemieniecki Feb 11 '19 at 14:52
if we want case insensitive substitution, we can put `replaced = re.sub('[ES]', 'a', s, flags=re.IGNORECASE)` – Venkataraman R Mar 22 '19 at 10:08
1

When use r before the search expression? – Timo Jan 22 '21 at 19:30
1

@Timo Regex often contains backslashes, which should not be escaped "by Python". https://stackoverflow.com/a/2081708/6371758 – Nicolai Weitkemper Mar 19 '21 at 11:29

score 20 · Answer 3 · answered Dec 06 '14 at 19:10

20

As a summary

import sys
import re

f = sys.argv[1]
find = sys.argv[2]
replace = sys.argv[3]
with open (f, "r") as myfile:
     s=myfile.read()
ret = re.sub(find,replace, s)   # <<< This is where the magic happens
print ret

answered Dec 06 '14 at 19:10

kpie

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score 11 · Answer 4 · answered May 23 '13 at 18:10

11

re.sub is definitely what you are looking for. And so you know, you don't need the anchors and the wildcards.

re.sub(r"(?i)interfaceOpDataFile", "interfaceOpDataFile %s" % filein, line)

will do the same thing--matching the first substring that looks like "interfaceOpDataFile" and replacing it.

answered May 23 '13 at 18:10

Nelz11

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I need to replace the entire line because the original file will have something like: `interfaceOpDataFile SomeDummyFile.txt` and I will want to replace it with: `interfaceOpDataFile SomeUsefulFile.txt` If I don't include the anchors how will replace know that I want to get rid of `SomeDummyFile.txt`? – Troy Rockwood May 23 '13 at 19:18
2

Ah, I mis-understood exactly what you were doing with the replacement. If each pair is on its own line, you still don't need the anchors explicity. `re.sub(r"(?i)(interfaceOpDataFile).*", r'\1 UsefulFile', line)` This will take the whole line, capture the arguement name, and add it back the the substitution for you. – Nelz11 May 23 '13 at 20:04

Python string.replace regular expression

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