RegEx to select a space between two digits

regex

Given a string as the following, containing only 0 and 1 separated by spaces with an additional space at the end of each line.
I want to select only the spaces between the digits but not the one at the end of the line to search and replace them with ,.
When I try to create a RegEx I either select all spaces or the surrounding digits too.

GIVEN      WANTED     NOT WANTED     TOTALLY NOT WANTED
0 1 0 1    0,1,0,1    0,1,0,1,       ,,,,
0 0 1 0    0,0,1,0    0,0,1,0,       ,,,,
1 0 1 0    1,0,1,0    1,0,1,0,       ,,,,
1 1 0 0    1,1,0,0    1,1,0,0,       ,,,,
1 1 1 0    1,1,1,0    1,1,1,0,       ,,,,
1 1 0 0    1,1,0,0    1,1,0,0,       ,,,,

So, how can I only select those spaces that have a digit on both sides, but not the digits. Is there some kind of syntax feature that matches but does not select?

Working answers:
As multiple answers were correct I want to collect them here. The accepted answer is the first one because it is the shortest regex and completely does what is needed.

(?!$) – please note there is a space at the beginning of this regex.

(?<=\d)\s(?=\d) or (?<=[01])\s(?=[01]) if your digits are either 0 or 1

Best Answer

if you're sure that your text contains only 1 and 0 you can use this

regex (?!$) there is a space character at the beginning of regex

check this Demo

and if you're not sure you can use this one (?<=\d) (?=\d) but this is going to work only if look ahead and look behind is supported , I don't know which language you're using exactly , this is PERL regex

check the Demo