Explanation of a regular expression

Here’s the regular expression to check for a valid IP:

function isValidIP(str) {
  return /^(([1-9]?\d|1\d\d|2[0-4]\d|25[0-5])(\.(?!$)|$)){4}$/.test(str);
}

Here’s the breakdown:

• ^ asserts the start of the string.
• (([1-9]?\d|1\d\d|2[0-4]\d|25[0-5])(\.(?!$)|$)){4} matches the IP address pattern for four octets.
  • ([1-9]?\d|1\d\d|2[0-4]\d|25[0-5]) matches a single octet value. Breaking it down further:
    • [1-9]?\d matches a single digit or a two-digit number that does not start with zero (range 1-99).
    • 1\d\d matches any three-digit number starting with ‘1’ (range 100-199).
    • 2[0-4]\d matches any three-digit number starting with ‘2’ and followed by a digit from 0 to 4 (range 200-249).
    • 25[0-5] matches any three-digit number starting with ’25’ and followed by a digit from 0 to 5 (range 250-255).
  • (\.(?!$)|$) matches the delimiter dot ‘.’ between octets or the end of the string:
    • \.(?!$) matches a dot ‘.’ that is not followed by the end of the string (negative lookahead).
    • $ matches the end of the string.
• {4} specifies that the pattern should occur exactly four times (matching four octets).
• $ asserts the end of the string.

The regular expression achieves the same as the following code which is much longer:

function isValidIP(str) {
  // check if number has four octets (less than four values)
  let array = str.split('.');
  let length = array.length;

  if (length != 4) {
    return false;
  }
  
  // create for loop
  for (let i = 0; i < length; i++){ 
    // check if numbers have leading zeros only if they're greater two digits
    if(array[i].length >= 2) {
      if(array[i].charAt(0) === '0' || array[i].charAt(0) === ' ' || array[i].endsWith(' ') || isNaN(array[i]) || array[i].includes('e') || array[i].includes('\n') || array[i].charAt(1) === ' ' || array[i] < 0) {
        return false;
      }
    }

    // check if length is zero
    if(array[i].length === 0) {
      return false;
    }
    
    // check if values are not numbers
    if(isNaN(array[i])) {
      return false;
    }
    // check if numbers are greater than 255
    if(array[i] > 255) {
      return false;
    }
  }
  return true;
}