Regular expressions:

var numberPattern = /\d+/g;

'something102asdfkj1948948'.match( numberPattern )

This would return an Array with two elements inside, '102' and '1948948'. Operate as you wish. If it doesn't match any it will return null.

To concatenate them:

'something102asdfkj1948948'.match( numberPattern ).join('')

Assuming you're not dealing with complex decimals, this should suffice I suppose.

I used the following regexp to extract numbers from a string:

var string = "border-radius:10px 20px 30px 40px";
var numbers = string.match(/\d+/g).map(Number);

For this specific example,

 var thenum = thestring.replace(/^\D+/g, ''); // Replace all leading non-digits with nothing

In the general case:

 thenum = "foo3bar5".match(/\d+/)[0] // "3"

Here's a bonus: regex generator.

function getre(str, num) {
  if(str === num)
    return 'nice try';
  var res = [/^\D+/g,/\D+$/g,/^\D+|\D+$/g,/\D+/g,/\D.*/g, /.*\D/g,/^\D+|\D.*$/g,/.*\D(?=\d)|\D+$/g];
  for(var i = 0; i < res.length; i++)
    if(str.replace(res[i], '') === num)
      return 'num = str.replace(/' + res[i].source + '/g, "")';
  return 'no idea';
};

function update() {
  $ = function(x) { return document.getElementById(x) };
  var re = getre($('str').value, $('num').value);
  $('re').innerHTML = 'Numex speaks: <code>' + re + '</code>';
}

<p>Hi, I'm Numex, the Number Extractor Oracle.
<p>What is your string? <input id="str" value="42abc"></p>
<p>What number do you want to extract? <input id="num" value="42"></p>
<p><button onclick="update()">Insert Coin</button></p>
<p id="re"></p>

If you want to not include the dot, a look-ahead would make sense:

/[-+]?\d*(\.(?=\d))?\d+/g

Another option is to move the second \d+ inside the parentheses:

/[-+]?\d+(\.\d+)?/g

Your RegEx works, if you want single digit right after /

Your regex \d+\/(\d)\d\d\d will match digits / then four digits and add first digit after slash in captured group. Note that 0th captured group will contain complete matched string. g flag is not necessary as there is only one instance of numbers in that pattern.

You can use this regex, but use first index to get the digit right after slash.

'1/2009'.match(/\d+\/(\d)\d\d\d/g)[1]
                                   ^  : Get me the value from first captured group.

And this regex can be optimized to below

.match(/\/(\d)/)[1]

This will match the number followed by /. And use the first captured group to extract the number.

<input type="text" onblur="console.log(this.value.match(/\/(\d)/)[1])" />

To get all digits after /

Just add + quantifier to \d in the captured group.

.match(/\/(\d+)/)[1]

Use a regular expression.

const r = /\d+/;
const s = "you can enter maximum 500 choices";
alert (s.match(r));

The expression \d+ means "one or more digits". Regular expressions by default are greedy meaning they'll grab as much as they can. Also, this:

const r = /\d+/;

is equivalent to:

const r = new RegExp("\\d+");

See the details for the RegExp object.

The above will grab the first group of digits. You can loop through and find all matches too:

const r = /\d+/g;
const s = "you can enter 333 maximum 500 choices";
const m;
while ((m = r.exec(s)) != null) {
  alert(m[0]);
}

The g (global) flag is key for this loop to work.

Here's an example function that will return an array representing the two numbers, or null if there wasn't a match:

function extractNumbers(str) {
    var m = /(\d+)to(\d+)/.exec(str);
    return m ? [+m[1], +m[2]] : null;
}

You can adapt that regular expression to suit your needs, say by making it case-insensitive: /(\d+)to(\d+)/i.exec(str).