gistfile1.txt

// This accepts a string numbuer and will increment the numeric part by 1
function stringNumber(str){
     // This will use a regular expression to match the last series of numbers that appear in 
     // your string (/\d+$/) and it will map that value to a function that will replace it with
     // the result of the actual number it contains (Number(n)), which will do something like 
     // mapping "999" to 999 and then incrementing it by 1 (Number(n) + 1)
     return str.replace(/\d+$/, function(n) { return Number(n) + 1; });
}

function stringNumber(str){
     // This essentially does the same thing as the previous version, except it can handle empty strings as
     // well by just appending a 1. Basically it tests if the string ends with a digit (/\d+$/.test(str)), 
     // if this is true, it will use the first clause of the inline if-statement (code between ? and :), which
     // performs the replacement you saw earlier. Otherwise, it will perform the second set (code after :, i.e.
     // str + '1', which will simply add '1' to the end of your string
     return /\d+$/.test(str) ? str.replace(/\d+$/, function(n) { return Number(n) + 1; }) : str + '1';
}

// The pad function will accept a number and a specific size to pad by (i.e the overall length of the result
function pad(num, size) {
  // Firstly, you build a large string by concatenating a series of zeroes (enough to handle the largest values 
  // you would need, with your number. You then would take a substring from the end of this to build your result
  // string (so if you had "0001" and used "0001".substr(-2), it would pull off the last two digits, yielding
  // "01"
  return ('0000000000' + num).substr(-size);
}

// This function puts everythign that you saw earlier together
function stringNumber(str) {
  // This will perform your replacement as mentioned earlier or simply append a '1' at the end of your string
  return /\d/.test(str) ? str.replace(/\d+$/, function(n) {
      // Since it's possilbe that the end of the number might change, you need to account for that when padding.
      // If you had 999, which is 3 characters long but were incrementing it, it would be 4 characters so we would
      // need to handle that when padding (as otherwise we could get 000 instead of 1000)
      var rangeToPad = /^9+$/.test(n) ? n.length + 1 : n.length;
      // Pad our result by the specified amount
      return pad(Number(n) + 1, rangeToPad);
    })
    // Or simply add a one to the value (padded by three digits)
    : str + pad(1, 3);
}