Change a text between two strings in Python with Regex

Question

I found several similar questions, but I cannot fit my problem to any of them. I try to find and replace a string between two other strings in a text.

reg = "%s(.*?)%s" % (str1,str2)
r = re.compile(reg,re.DOTALL)
result = r.sub(newstring, originaltext)

The problem is that the code above replace also str1 and str2, whereas I want to replace only the text between them. Something obviously that I miss?

Update:

I simplified example:

text = 'abcdefghijklmnopqrstuvwxyz'

str1 = 'gh'
str2 = 'op'

newstring = 'stackexchange'

reg = "%s(.*?)%s" % (str1,str2)
r = re.compile(reg,re.DOTALL)
result = r.sub(newstring, text)

print result

The result is abcdefstackexchangeqrstuvwxyz whereas I need abcdefghstackexchangeopqrstuvwxyz

Answer 1

Use a combination of lookarounds in your regular expression.

reg = "(?<=%s).*?(?=%s)" % (str1,str2)

Explanation:

Lookarounds are zero-width assertions. They don't consume any characters on the string.

(?<=    # look behind to see if there is:
  gh    #   'gh'
)       # end of look-behind
.*?     # any character except \n (0 or more times)
(?=     # look ahead to see if there is:
  op    #   'op'
)       # end of look-ahead

Working Demo