This function formats UK phone numbers, adds missing national trunk codes, and returns an error message if invalid.

format_uk_phone_number.js

/**

format_uk_phone_number - A function to format UK phone numbers
This function takes in a phone number string and formats it according to UK phone number conventions.
@param {string} phone_number - The phone number to be formatted.
@returns {string} The formatted UK phone number string, or "Invalid UK phone number" if the input is invalid.

**/

function format_uk_phone_number(phone_number) {
    // Remove any non-digit characters
    phone_number = phone_number.replace(/\D/g, '');

    // Check the length of the input string
    let len = phone_number.length;

    if (len < 3 || (len >= 12 && phone_number.substring(0, 3) !== "+44")) {
        return "Invalid UK phone number";
    }

    // Check if the phone number includes the national trunk code (i.e. starts with "0").
    if (phone_number[0] !== "0") {
        phone_number = "0" + phone_number;
    }

    let code = phone_number.substring(0, 2);

    if (code === "01" || code === "02") {
        let area_codes = {
            "020": [3, 4, 4], // London
            "023": [3, 4, 4], // Southampton
            "024": [3, 4, 4], // Coventry
            "028": [3, 4, 4], // Northern Ireland
            "029": [3, 4, 4], // Cardiff
        };

        let formatted = "";
        let area_code = phone_number.substring(0, 3);
        if (area_codes.hasOwnProperty(area_code)) {
            let split = area_codes[area_code];
            formatted = phone_number.substring(0, split[0]) + " " + phone_number.substring(split[0], split[0] + split[1]) + " " + phone_number.substring(split[0] + split[1], split[0] + split[1] + split[2]);
        } else {
            formatted = phone_number.substring(0, 5) + " " + phone_number.substring(5, 11);
        }
        return formatted;
    }

    if (code === "03") {
        return phone_number.substring(0, 4) + " " + phone_number.substring(4, 7) + " " + phone_number.substring(7);
    }

    if (["05", "06", "07", "08"].includes(code)) {
        // Non-geographic and premium rate numbers
        if (len == 7 || len == 8) {
            if (phone_number.substring(0, 4) == "0800") {
                return phone_number.substring(0, 4) + " " + phone_number.substring(4);
            } else {
                return phone_number.substring(0, 3) + " " + phone_number.substring(3);
            }
        }
        if (len == 10) {
            return phone_number.substring(0, 4) + " " + phone_number.substring(4, 7) + " " + phone_number.substring(7);
        }
        if (len == 11) {
            return phone_number.substring(0, 5) + " " + phone_number.substring(5, 8) + " " + phone_number.substring(8);
        }
    }

    if (len <= 6) {
        return phone_number;
    }

    return "Invalid UK " + len + " " + phone_number;
}

format_uk_phone_number.php

<?php

function format_uk_phone_number($phone_number) {
  // Remove any non-digit characters
  $phone_number = preg_replace('/\D/', '', $phone_number);

  // Check the length of the input string
  $len = strlen($phone_number);

  if ($len < 3 || ($len >= 12 && substr($phone_number, 0, 3) !== "+44")) {
      return "Invalid UK phone number";
  }

  // Check if the phone number includes the national trunk code (i.e. starts with "0").
  if ($phone_number[0] !== "0") {
      $phone_number = "0" . $phone_number;
  }

  $code = substr($phone_number, 0, 2);

  if ($code === "01" || $code === "02") {
      $area_codes = [
          "020" => [3, 4, 4], // London
          "023" => [3, 4, 4], // Southampton
          "024" => [3, 4, 4], // Coventry
          "028" => [3, 4, 4], // Northern Ireland
          "029" => [3, 4, 4], // Cardiff
      ];

      $formatted = "";
      $area_code = substr($phone_number, 0, 3);
      if (isset($area_codes[$area_code])) {
          $split = $area_codes[$area_code];
          $formatted = substr($phone_number, 0, $split[0]) . " " . substr($phone_number, $split[0], $split[1]) . " " . substr($phone_number, $split[0] + $split[1], $split[2]);
      } else {
          $formatted = substr($phone_number, 0, 5) . " " . substr($phone_number, 5, 6);
      }
      return $formatted;
  }

  if ($code === "03") {
      return substr($phone_number, 0, 4) . " " . substr($phone_number, 4, 3) . " " . substr($phone_number, 7);
  }

  if (in_array($code, ["05", "06", "07", "08"])) {
      // Non-geographic and premium rate numbers
      if ($len == 7 || $len == 8) {
          if (substr($phone_number, 0, 4) == "0800") {
              return substr($phone_number, 0, 4) . " " . substr($phone_number, 4);
          } else {
              return substr($phone_number, 0, 3) . " " . substr($phone_number, 3);
          }
      }                                        
      if ($len == 10) {
          return substr($phone_number, 0, 4) . " " . substr($phone_number, 4, 3) . " " . substr($phone_number, 7);
      }
      if ($len == 11) {
          return substr($phone_number, 0, 5) . " " . substr($phone_number, 5, 3) . " " . substr($phone_number, 8);
      }
  }

  if ($len <= 6) {
      return $phone_number;
  }

  return "Invalid UK " . $len . " " . $phone_number;
}

?>