Recursively patten js

I want to check a recursively text that verufy three rules.

1º: All the string should be a sequence of numbers between 0-31 + a dot .

Example: 1.23.5.12

2º: The string can't begin or end with a dot. Like this. .1.23.5.12.

3º You can write a max of 51 digits (following the previous rules)

I tried to make a pattern to my js function. But this dont work.

This is my function:

  var str = document.getElementById("numero").value;
        var patt1 = /^[0-9]+\./g;
        var result = str.match(patt1);
        document.getElementById("demo").innerHTML = result;

What is wrong in the pattern?