Why are there different regex results in javascript and python?

I am trying to match 5 letter words that have ‘a’ as the second character, followed by two double letters. e.g. ‘bally’

I’m using regex (so now have 2 problems).

In python, this is not matching:

import re
caxxy = re.compile('^[a-z]a([a-z])\1y')
print (caxxy.match('bally')) #None

However, in javascript, this is matching:

const word = 'bally';
const caxxy = /^[a-z]a([a-z])\1y/;
const match = word.match(caxxy);
console.log(match); // > Array ["bally", "l"]

Why is the same string and regex returning different results in python and js?

>Solution :

It has nothing to do with different programming languages. It’s a difference between a RegExp literal and a string.

'^[a-z]a([a-z])\1y'

is a string containing

^[a-z]a([a-z])1y

The strings

'\1'

and

'1'

are similar. It’s explained here.
You have to escape the backslash with

'^[a-z]a([a-z])\\1y'

or use raw strings

r'^[a-z]a([a-z])\1y'

On the other side,

/^[a-z]a([a-z])\1y/

isn’t a string. It’s a RegExp literal. You don’t have to escape the backslash.

You have the same behavior in JavaScript using strings:

const word = 'bally';
let caxxy = new RegExp('^[a-z]a([a-z])\1y');
let match = word.match(caxxy);
console.log(match);
caxxy = new RegExp('^[a-z]a([a-z])\\1y');
match = word.match(caxxy);
console.log(match);