Efficient repeated substring replace with Python

Question

How can I efficiently replace substrings in python, where one substring may be part of another? For example:

>>> "||||".replace("||","|X|")
'|X||X|'
# What I want: |X|X|X|

Certainly I could keep repeating the replace and until there are no more instances of || in the string, but surely there's a better/faster way?

• John Ruddell: "||||".replace("|","|X") + '|' so just this then?
• Thomas Johnson: That gives '|X|X|X|X|' which adds an extra X
• user2357112: It seems like it might be better to reformulate your desired goal in terms other than overlapping substring replacement. For example, here, it looks like you want to insert text between delimiters, so something like a regex replace with lookahead and lookbehind assertions for a single | each would work.
• user2357112: What output would you want for 'aaaa'.replace('aaa', 'pony')? There are two overlapping instances of 'aaa' in the input, but it doesn't really make sense to replace both of them.
• l'L'l: "||||".replace("||","|X|X").strip('X')

Answer 1

In general you need to repeat the process.

In this specific case however you can use a regexp to insert X between consecutive | signs:

import re
print(re.sub("[|](?=[|])",
 "|X",
 "||||"))

meaning is "replace any pipe with pipe+X if what follows is another pipe (but don't consider it part of the match)"