I have two regex in .net preRegex = "(?<=(^|[^A-Za-z0-9]+))" postRegex = "(?=([^A-Za-z0-9]+)|$)" . What is the alternate of it in python?

Question

I have two regex strings in .net

preRegex = "(?<=(^|[^A-Za-z0-9]+))"
postRegex = "(?=([^A-Za-z0-9]+)|$)"

I want their alternative in python. My problem is let say I have a string

s="I am in aeroplane."

What I need is to replace "aeroplane with aeroPlain" so I want to make regex like

newKey = preRegex + aeroplane + postRegex
pattern = re.compile(newKey,re.IGNORECASE)

new regex string look like

(?<=(^|[^A-Za-z0-9]+))aeroplane(?=([^A-Za-z0-9]+)|$)

but it is giving me error "look-behind requires fixed-width pattern". I am new to python, Help would be appreciated. Thanks

As the question stands, I'm not sure what the objective is. It may be helpful to include .NET code that has the working regular expression. It may also be helpful to include the generalized objective. E.g. "looking to replace all instances of aeroplane with aeroPlain in a string". — Arbiter of buffoonery, Dec 07 '17 at 07:10
The error is crystal clear: `Python lookbehind assertions need to be fixed width`, on top of this in your current example your pattern is at the end of the string so you could replace it by `(?=\.|$)` — Allan, Dec 07 '17 at 07:11

score 0 · Answer 1 · answered Dec 07 '17 at 07:18

0

You can use the following regex:

(^|[^A-Za-z0-9]+)aeroplane([^A-Za-z0-9]+|$)

and when you replace, you can call the back reference to the first and second part of your regex to fetch their value.

Replacement string will be something like '\1aeroPlain\2'. For more information on backref in python:https://docs.python.org/3/howto/regex.html

Good luck!

answered Dec 07 '17 at 07:18

Allan

1

Your answer is not quite right since what you suggest won't work exactly the same in case of overlapping matches. – Wiktor Stribiżew Dec 07 '17 at 07:24
could you develop a bit more? Sensei :-) – Allan Dec 07 '17 at 07:25

1 Answers1