Question

Consider a basic regex like a(.+?)a. How can one replace all occurences of that regex in a file with the content of the first group?

Answer 1

Use can use the re module to use regular expressions in python and the fileinput module to simply replace text in files in-place

---

Example:

import fileinput
import re

fn = "test.txt" # your filename

r = re.compile('a(.+?)a')
for line in fileinput.input(fn, inplace=True):
  match = r.match(line)
  print match.group() if match else line.replace('\n', '')

---

Before:

hello this
aShouldBeAMatch!!!!! and this should be gone
you know

After:

hello this
aShouldBeAMa
you know

---

Note: this works because the argument inplace=True causes input file to be moved to a backup file and standard output is directed to the input file, as documented under Optional in-place filtering.