Python string.replace regular expression

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?

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

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

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

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.