Difference between perl regex to add dots between two numbers in Moses

Context, I'm trying to port a Perl code into Python from https://github.com/moses-smt/mosesdecoder/blob/master/scripts/tokenizer/normalize-punctuation.perl#L87 and there is this regex here in Perl:

s/(\d) (\d)/$1.$2/g;

If I try it with the Perl script given the input text 123 45, it returns the same string with a dot. As a sanity check, I've tried on the command line too:

echo "123 45" | perl -pe 's/(\d) (\d)/$1.$2/g;'

[out]:

123.45

And it does so too when I convert the regex to Python,

>>> import re
>>> r, s = r'(\d) (\d)', '\g<1>.\g<2>'
>>> print(re.sub(r, s, '123 45'))
123.45

But when I use the Moses script:

$ wget https://raw.githubusercontent.com/moses-smt/mosesdecoder/master/scripts/tokenizer/normalize-punctuation.perl
--2019-03-19 12:33:09--  https://raw.githubusercontent.com/moses-smt/mosesdecoder/master/scripts/tokenizer/normalize-punctuation.perl
Resolving raw.githubusercontent.com... 151.101.0.133, 151.101.64.133, 151.101.128.133, ...
Connecting to raw.githubusercontent.com|151.101.0.133|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 905 [text/plain]
Saving to: 'normalize-punctuation.perl'

normalize-punctuation.perl    100%[================================================>]     905  --.-KB/s    in 0s      

2019-03-19 12:33:09 (8.72 MB/s) - 'normalize-punctuation.perl' saved [1912]

$ echo "123 45" > foobar

$ perl normalize-punctuation.perl < foobar
123 45

Even when we try to print the string before and after the regex in the Moses code, i.e.

if ($language eq "de" || $language eq "es" || $language eq "cz" || $language eq "cs" || $language eq "fr") {
    s/(\d) (\d)/$1,$2/g;
    }
else {
    print $_;
    s/(\d) (\d)/$1.$2/g;
    print $_;
    }

[out]:

123 45
123 45
123 45

We see that before and after the regex, there's no change in the string.

My question in parts are:

Is the Python \g<1>.\g<2> regex equivalent to the Perl's $1.$2?
Why is it that the Perl regex didn't add the full stop . between the two digit groups in Moses?
How to replicate Perl's behavior in Moses in Python regex?
How to replicate Python's behavior in Perl regex in Moses?

Solution

The reason why this code from moose doesn't work is because it search for non-breaking space, not just space. It is not easy to see, but hexdump could help you with that:

fe-laptop-p:moose fe$ head -n87 normalize-punctuation.perl | tail -n1 | hexdump -C
00000000  09 73 2f 28 5c 64 29 c2  a0 28 5c 64 29 2f 24 31  |.s/(\d)..(\d)/$1|
00000010  2e 24 32 2f 67 3b 0a                              |.$2/g;.|
00000017
fe-laptop-p:moose fe$ head -n87 normalize-punctuation.perl.with_space | tail -n1 | hexdump -C
00000000  09 73 2f 28 5c 64 29 20  28 5c 64 29 2f 24 31 2e  |.s/(\d) (\d)/$1.|
00000010  24 32 2f 67 3b 0a                                 |$2/g;.|
00000016

See the difference: c2 a0 vs 20?

p.s. as for comments about adding plus sign to regex: it is not needed here, as it is enough to put dot sign between two adjacent digits and no need to find full numbers