Implement an input with a mask

I would like to implement a mask for a text input field which accepts a date. The masked value should display directly inside of the input.

Something like this:

<input type='text' value='____/__/__'>

I wrote the mask as a value in that example, but my intent is to allow people to write a date without typing / or - to separate months, years and days. The user should be able to enter numbers into the displayed field, while the mask enforces the format automatically as the user types.

I have seen this behavior on other sites, but I have no idea how it works or how to implement it myself.

Input masks can be implemented using a combination of the keyup event, and the HTMLInputElement value, selectionStart, and selectionEnd properties. Here's a very simple implementation which does some of what you want. It's certainly not perfect, but works well enough to demonstrate the principle:

Array.prototype.forEach.call(document.body.querySelectorAll("*[data-mask]"), applyDataMask);

function applyDataMask(field) {
    var mask = field.dataset.mask.split('');
    
    // For now, this just strips everything that's not a number
    function stripMask(maskedData) {
        function isDigit(char) {
            return /\d/.test(char);
        }
        return maskedData.split('').filter(isDigit);
    }
    
    // Replace `_` characters with characters from `data`
    function applyMask(data) {
        return mask.map(function(char) {
            if (char != '_') return char;
            if (data.length == 0) return char;
            return data.shift();
        }).join('')
    }
    
    function reapplyMask(data) {
        return applyMask(stripMask(data));
    }
    
    function changed() {   
        var oldStart = field.selectionStart;
        var oldEnd = field.selectionEnd;
        
        field.value = reapplyMask(field.value);
        
        field.selectionStart = oldStart;
        field.selectionEnd = oldEnd;
    }
    
    field.addEventListener('click', changed)
    field.addEventListener('keyup', changed)
}

ISO Date: <input type="text" value="____-__-__" data-mask="____-__-__"/><br/>
Telephone: <input type="text" value="(___) ___-____" data-mask="(___) ___-____"/><br/>

(View in JSFiddle)

There are also a number of libraries out there which perform this function. Some examples include:

• jquery.inputmask
• MASKED INPUT PLUGIN
• Politespace (presents an alternative to input masks)

A solution that responds to the input event instead of key events (like keyup) will give a smooth experience (no wiggles), and also works when changes are made without the keyboard (context menu, mouse drag, other device...).

The code below will look for input elements that have both a placeholder attribute and a data-slots attribute. The latter should define the character(s) in the placeholder that is/are intended as input slot, for example, "_". An optional data-accept attribute can be provided with a regular expression that defines which characters are allowed in such a slot. The default is \d, i.e. digits.

// This code empowers all input tags having a placeholder and data-slots attribute
document.addEventListener('DOMContentLoaded', () => {
    for (const el of document.querySelectorAll("[placeholder][data-slots]")) {
        const pattern = el.getAttribute("placeholder"),
            slots = new Set(el.dataset.slots || "_"),
            prev = (j => Array.from(pattern, (c,i) => slots.has(c)? j=i+1: j))(0),
            first = [...pattern].findIndex(c => slots.has(c)),
            accept = new RegExp(el.dataset.accept || "\\d", "g"),
            clean = input => {
                input = input.match(accept) || [];
                return Array.from(pattern, c =>
                    input[0] === c || slots.has(c) ? input.shift() || c : c
                );
            },
            format = () => {
                const [i, j] = [el.selectionStart, el.selectionEnd].map(i => {
                    i = clean(el.value.slice(0, i)).findIndex(c => slots.has(c));
                    return i<0? prev[prev.length-1]: back? prev[i-1] || first: i;
                });
                el.value = clean(el.value).join``;
                el.setSelectionRange(i, j);
                back = false;
            };
        let back = false;
        el.addEventListener("keydown", (e) => back = e.key === "Backspace");
        el.addEventListener("input", format);
        el.addEventListener("focus", format);
        el.addEventListener("blur", () => el.value === pattern && (el.value=""));
    }
});

[data-slots] { font-family: monospace }

<label>Date time: 
    <input placeholder="dd/mm/yyyy hh:mm" data-slots="dmyh">
</label><br>
<label>Telephone:
    <input placeholder="+1 (___) ___-____" data-slots="_">
</label><br>
<label>MAC Address:
    <input placeholder="XX:XX:XX:XX:XX:XX" data-slots="X" data-accept="[\dA-H]">
</label><br>
<label>Alphanumeric:
    <input placeholder="__-__-__-____" data-slots="_" data-accept="\w" size="13">
</label><br>
<label>Credit Card:
    <input placeholder=".... .... .... ...." data-slots="." data-accept="\d" size="19">
</label><br>

I did my own implementation using the following principles:

1. allow entry of only numbers. (keypress event)
2. get all numbers in an array
3. replace every "_" character of the mask by a number from the array in a loop

Improvements are welcome.

/**
 * charCode [48,57] 	Numbers 0 to 9
 * keyCode 46  			"delete"
 * keyCode 9  			"tab"
 * keyCode 13  			"enter"
 * keyCode 116 			"F5"
 * keyCode 8  			"backscape"
 * keyCode 37,38,39,40	Arrows
 * keyCode 10			(LF)
 */
function validate_int(myEvento) {
  if ((myEvento.charCode >= 48 && myEvento.charCode <= 57) || myEvento.keyCode == 9 || myEvento.keyCode == 10 || myEvento.keyCode == 13 || myEvento.keyCode == 8 || myEvento.keyCode == 116 || myEvento.keyCode == 46 || (myEvento.keyCode <= 40 && myEvento.keyCode >= 37)) {
    dato = true;
  } else {
    dato = false;
  }
  return dato;
}

function phone_number_mask() {
  var myMask = "(___) ___-____";
  var myCaja = document.getElementById("phone");
  var myText = "";
  var myNumbers = [];
  var myOutPut = ""
  var theLastPos = 1;
  myText = myCaja.value;
  //get numbers
  for (var i = 0; i < myText.length; i++) {
    if (!isNaN(myText.charAt(i)) && myText.charAt(i) != " ") {
      myNumbers.push(myText.charAt(i));
    }
  }
  //write over mask
  for (var j = 0; j < myMask.length; j++) {
    if (myMask.charAt(j) == "_") { //replace "_" by a number 
      if (myNumbers.length == 0)
        myOutPut = myOutPut + myMask.charAt(j);
      else {
        myOutPut = myOutPut + myNumbers.shift();
        theLastPos = j + 1; //set caret position
      }
    } else {
      myOutPut = myOutPut + myMask.charAt(j);
    }
  }
  document.getElementById("phone").value = myOutPut;
  document.getElementById("phone").setSelectionRange(theLastPos, theLastPos);
}

document.getElementById("phone").onkeypress = validate_int;
document.getElementById("phone").onkeyup = phone_number_mask;

<input type="text" name="phone" id="phone" placeholder="(123) 456-7890" required="required" title="e.g (123) 456-7890" pattern="^\([0-9]{3}\)\s[0-9]{3}-[0-9]{4}$">

You can achieve this also by using JavaScripts's native method. Its pretty simple and doesn't require any extra library to import.

<input type="text" name="date" placeholder="yyyy-mm-dd" onkeyup="
  var date = this.value;
  if (date.match(/^\d{4}$/) !== null) {
     this.value = date + '-';
  } else if (date.match(/^\d{4}\-\d{2}$/) !== null) {
     this.value = date + '-';
  }" maxlength="10">

You can also try my implementation, which doesn't have delay after each key press when typing the contents, and has full support for backspace and delete.

You can try it online: https://jsfiddle.net/qmyo6a1h/1/

	<html>
	<style>
	input{
	  font-family:'monospace';
	}
	</style>
	<body>
	  <input type="text" id="phone" placeholder="123-5678-1234" title="123-5678-1234" input-mask="___-____-____">
	  <input type="button" onClick="showValue_phone()" value="Show Value" />
	  <input type="text" id="console_phone" />
	  <script>
		function InputMask(element) {
		  var self = this;

		  self.element = element;

		  self.mask = element.attributes["input-mask"].nodeValue;

		  self.inputBuffer = "";

		  self.cursorPosition = 0;

		  self.bufferCursorPosition = 0;

		  self.dataLength = getDataLength();

		  function getDataLength() {
			var ret = 0;

			for (var i = 0; i < self.mask.length; i++) {
			  if (self.mask.charAt(i) == "_") {
				ret++;
			  }
			}

			return ret;
		  }

		  self.keyEventHandler = function (obj) {
			obj.preventDefault();

			self.updateBuffer(obj);
			self.manageCursor(obj);
			self.render();
			self.moveCursor();
		  }

		  self.updateBufferPosition = function () {
			var selectionStart = self.element.selectionStart;
			self.bufferCursorPosition = self.displayPosToBufferPos(selectionStart);
			console.log("self.bufferCursorPosition==" + self.bufferCursorPosition);
		  }

		  self.onClick = function () {
			self.updateBufferPosition();
		  }

		  self.updateBuffer = function (obj) {
			if (obj.keyCode == 8) {
			  self.inputBuffer = self.inputBuffer.substring(0, self.bufferCursorPosition - 1) + self.inputBuffer.substring(self.bufferCursorPosition);
			}
			else if (obj.keyCode == 46) {
			  self.inputBuffer = self.inputBuffer.substring(0, self.bufferCursorPosition) + self.inputBuffer.substring(self.bufferCursorPosition + 1);
			}
			else if (obj.keyCode >= 37 && obj.keyCode <= 40) {
			  //do nothing on cursor keys.
			}
			else {
			  var selectionStart = self.element.selectionStart;
			  var bufferCursorPosition = self.displayPosToBufferPos(selectionStart);
			  self.inputBuffer = self.inputBuffer.substring(0, bufferCursorPosition) + String.fromCharCode(obj.which) + self.inputBuffer.substring(bufferCursorPosition);
			  if (self.inputBuffer.length > self.dataLength) {
				self.inputBuffer = self.inputBuffer.substring(0, self.dataLength);
			  }
			}
		  }

		  self.manageCursor = function (obj) {
			console.log(obj.keyCode);
			if (obj.keyCode == 8) {
			  self.bufferCursorPosition--;
			}
			else if (obj.keyCode == 46) {
			  //do nothing on delete key.
			}
			else if (obj.keyCode >= 37 && obj.keyCode <= 40) {
			  if (obj.keyCode == 37) {
				self.bufferCursorPosition--;
			  }
			  else if (obj.keyCode == 39) {
				self.bufferCursorPosition++;
			  }
			}
			else {
			  var bufferCursorPosition = self.displayPosToBufferPos(self.element.selectionStart);
			  self.bufferCursorPosition = bufferCursorPosition + 1;
			}
		  }

		  self.setCursorByBuffer = function (bufferCursorPosition) {
			var displayCursorPos = self.bufferPosToDisplayPos(bufferCursorPosition);
			self.element.setSelectionRange(displayCursorPos, displayCursorPos);
		  }

		  self.moveCursor = function () {
			self.setCursorByBuffer(self.bufferCursorPosition);
		  }

		  self.render = function () {
			var bufferCopy = self.inputBuffer;
			var ret = {
			  muskifiedValue: ""
			};

			var lastChar = 0;

			for (var i = 0; i < self.mask.length; i++) {
			  if (self.mask.charAt(i) == "_" &&
				bufferCopy) {
				ret.muskifiedValue += bufferCopy.charAt(0);
				bufferCopy = bufferCopy.substr(1);
				lastChar = i;
			  }
			  else {
				ret.muskifiedValue += self.mask.charAt(i);
			  }
			}

			self.element.value = ret.muskifiedValue;

		  }

		  self.preceedingMaskCharCount = function (displayCursorPos) {
			var lastCharIndex = 0;
			var ret = 0;

			for (var i = 0; i < self.element.value.length; i++) {
			  if (self.element.value.charAt(i) == "_"
				|| i > displayCursorPos - 1) {
				lastCharIndex = i;
				break;
			  }
			}

			if (self.mask.charAt(lastCharIndex - 1) != "_") {
			  var i = lastCharIndex - 1;
			  while (self.mask.charAt(i) != "_") {
				i--;
				if (i < 0) break;
				ret++;
			  }
			}

			return ret;
		  }

		  self.leadingMaskCharCount = function (displayIndex) {
			var ret = 0;

			for (var i = displayIndex; i >= 0; i--) {
			  if (i >= self.mask.length) {
				continue;
			  }
			  if (self.mask.charAt(i) != "_") {
				ret++;
			  }
			}

			return ret;
		  }

		  self.bufferPosToDisplayPos = function (bufferIndex) {
			var offset = 0;
			var indexInBuffer = 0;

			for (var i = 0; i < self.mask.length; i++) {
			  if (indexInBuffer > bufferIndex) {
				break;
			  }

			  if (self.mask.charAt(i) != "_") {
				offset++;
				continue;
			  }

			  indexInBuffer++;
			}
			var ret = bufferIndex + offset;

			return ret;
		  }

		  self.displayPosToBufferPos = function (displayIndex) {
			var offset = 0;
			var indexInBuffer = 0;

			for (var i = 0; i < self.mask.length && i <= displayIndex; i++) {
			  if (indexInBuffer >= self.inputBuffer.length) {
				break;
			  }

			  if (self.mask.charAt(i) != "_") {
				offset++;
				continue;
			  }

			  indexInBuffer++;
			}

			return displayIndex - offset;
		  }

		  self.getValue = function () {
			return this.inputBuffer;
		  }
		  self.element.onkeypress = self.keyEventHandler;
		  self.element.onclick = self.onClick;
		}

		function InputMaskManager() {
		  var self = this;

		  self.instances = {};

		  self.add = function (id) {
			var elem = document.getElementById(id);
			var maskInstance = new InputMask(elem);
			self.instances[id] = maskInstance;
		  }

		  self.getValue = function (id) {
			return self.instances[id].getValue();
		  }

		  document.onkeydown = function (obj) {
			if (obj.target.attributes["input-mask"]) {
			  if (obj.keyCode == 8 ||
				obj.keyCode == 46 ||
				(obj.keyCode >= 37 && obj.keyCode <= 40)) {

				if (obj.keyCode == 8 || obj.keyCode == 46) {
				  obj.preventDefault();
				}

				//needs to broadcast to all instances here:
				var keys = Object.keys(self.instances);
				for (var i = 0; i < keys.length; i++) {
				  if (self.instances[keys[i]].element.id == obj.target.id) {
					self.instances[keys[i]].keyEventHandler(obj);
				  }
				}
			  }
			}
		  }
		}

		//Initialize an instance of InputMaskManager and
		//add masker instances by passing in the DOM ids
		//of each HTML counterpart.
		var maskMgr = new InputMaskManager();
		maskMgr.add("phone");

		function showValue_phone() {
		  //-------------------------------------------------------__Value_Here_____
		  document.getElementById("console_phone").value = maskMgr.getValue("phone");
		}
	  </script>
	</body>

	</html>

I wrote a similar solution some time ago.
Of course it's just a PoC and can be improved further.

This solution covers the following features:

• Seamless character input
• Pattern customization
• Live validation while you typing
• Full date validation (including correct days in each month and a leap year consideration)
• Descriptive errors, so the user will understand what is going on while he is unable to type a character
• Fix cursor position and prevent selections
• Show placeholder if the value is empty

const patternFreeChar = "_";
const dayValidator = [/^[0-3]$/, /^0[1-9]|[12]\d|3[01]$/];
const monthValidator = [/^[01]$/, /^0[1-9]|1[012]$/];
const yearValidator = [/^[12]$/, /^19|20$/, /^(19|20)\d$/, /^(19|20)\d\d$/];

/**
 * Validate a date as your type.
 * @param {string} date The date in the provided format as a string representation.
 * @param {string} format The format to use.
 * @throws {Error} When the date is invalid.
 */
function validateStartTypingDate(date, format='DDMMYYYY') {
  if ( !date ) return "";

  date = date.substr(0, 8);

  if ( !/^\d+$/.test(date) )
    throw new Error("Please type numbers only");

  const formatAsArray = format.split('');
  const dayIndex = formatAsArray.findIndex(c => c == 'D');
  const monthIndex = formatAsArray.findIndex(c => c == 'M');
  const yearIndex = formatAsArray.findIndex(c => c == 'Y');

  const dayStr = date.substr(dayIndex,2);
  const monthStr = date.substr(monthIndex,2);
  const yearStr = date.substr(yearIndex,4);

  if ( dayStr && !dayValidator[dayStr.length-1].test(dayStr) ) {
    switch (dayStr.length) {
      case 1:
        throw new Error("Day in month can start only with 0, 1, 2 or 3");
      case 2:
        throw new Error("Day in month must be in a range between 01 and 31");
    }
  }

  if ( monthStr && !monthValidator[monthStr.length-1].test(monthStr) ) {
    switch (monthStr.length) {
      case 1:
        throw new Error("Month can start only with 0 or 1");
      case 2:
        throw new Error("Month number must be in a range between 01 and 12");
    }
  }

  if ( yearStr && !yearValidator[yearStr.length-1].test(yearStr) ) {
    switch (yearStr.length) {
      case 1:
        throw new Error("We support only years between 1900 and 2099, so the full year can start only with 1 or 2");
      default:
        throw new Error("We support only years between 1900 and 2099, so the full year can start only with 19 or 20");
    }
  }

  const day = parseInt(dayStr);
  const month = parseInt(monthStr);
  const year = parseInt(yearStr);
  const monthName = new Date(0,month-1).toLocaleString('en-us',{month:'long'});

  if ( day > 30 && [4,6,9,11].includes(month) )
    throw new Error(`${monthName} have maximum 30 days`);

  if ( day > 29 && month === 2 )
    throw new Error(`${monthName} have maximum 29 days`);

  if ( date.length === 8 ) {
    if ( !isLeap(year) && month === 2 && day === 29 )
      throw new Error(`The year you are trying to enter (${year}) is not a leap year. Thus, in this year, ${monthName} can have maximum 28 days`);
  }

  return date;
}

/**
 * Check whether the given year is a leap year.
 */
function isLeap(year) {
  return new Date(year, 1, 29).getDate() === 29;
}

/**
 * Move cursor to the end of the provided input element.
 */
function moveCursorToEnd(el) {
  if (typeof el.selectionStart == "number") {
    el.selectionStart = el.selectionEnd = el.value.length;
  } else if (typeof el.createTextRange != "undefined") {
    el.focus();
    var range = el.createTextRange();
    range.collapse(false);
    range.select();
  }
}

/**
 * Move cursor to the end of the self input element.
 */
function selfMoveCursorToEnd() {
  return moveCursorToEnd(this);
}

const inputs = document.querySelectorAll("input");

inputs.forEach(input => {
  const { format, pattern } = input.dataset;
  input.addEventListener("keydown", function(event){
    event.preventDefault();
    document.getElementById("date-error-msg").innerText = "";

    // On digit pressed
    let inputMemory = this.dataset.inputMemory || "";

    if ( event.key.length === 1 ) {
      try {
        inputMemory = validateStartTypingDate(inputMemory + event.key, format);
      } catch (err) {
        document.getElementById("date-error-msg").innerText = err.message;
      }
    }

    // On backspace pressed
    if ( event.code === "Backspace" ) {
      inputMemory = inputMemory.slice(0, -1);
    }

    // Build an output using a pattern
    if ( this.dataset.inputMemory !== inputMemory ) {
      let output = pattern;
      for ( let i=0, digit; i<inputMemory.length, digit=inputMemory[i]; i++ ) {
        output = output.replace(patternFreeChar, digit);
      }
      this.dataset.inputMemory = inputMemory;
      this.value = output;
    }

    // Clean the value if the memory is empty
    if ( inputMemory === "" ) {
      this.value = "";
    }
  }, false);

  input.addEventListener('select', selfMoveCursorToEnd, false);
  input.addEventListener('mousedown', selfMoveCursorToEnd, false);
  input.addEventListener('mouseup', selfMoveCursorToEnd, false);
  input.addEventListener('click', selfMoveCursorToEnd, false);
});

input {
  width: 250px;
}

<div><input type="text" placeholder="DD/MM/YYYY" data-format="DDMMYYYY" data-pattern="__/__/____" /></div>
<div><input type="text" placeholder="MM/DD/YYYY" data-format="MMDDYYYY" data-pattern="__/__/____" /></div>
<div><input type="text" placeholder="YYYY-MM-DD" data-format="YYYYMMDD" data-pattern="____-__-__" /></div>
<div><input type="text" placeholder="Day: DD, Year: YYYY, Month: MM" data-format="DDYYYYMM" data-pattern="Day: __, Year: ____, Month: __" /></div>
<div id="date-error-msg"></div>

A link to jsfiddle: https://jsfiddle.net/sm3xw61n/2/

Good luck!

Use this to implement mask:

https://cdnjs.cloudflare.com/ajax/libs/jquery.inputmask/5.0.6/jquery.inputmask.min.js

<input id="phone_number" class="ant-input" type="text" placeholder="(XXX) XXX-XXXX" data-inputmask-mask="(999) 999-9999">



jQuery("#phone_number").inputmask({"mask": "(999) 999-9999"});

Array.prototype.forEach.call(document.body.querySelectorAll("*[data-mask]"), applyDataMask);

function applyDataMask(field) {
    var mask = field.dataset.mask.split('');
    
    // For now, this just strips everything that's not a number
    function stripMask(maskedData) {
        function isDigit(char) {
            return /\d/.test(char);
        }
        return maskedData.split('').filter(isDigit);
    }
    
    // Replace `_` characters with characters from `data`
    function applyMask(data) {
        return mask.map(function(char) {
            if (char != '_') return char;
            if (data.length == 0) return char;
            return data.shift();
        }).join('')
    }
    
    function reapplyMask(data) {
        return applyMask(stripMask(data));
    }
    
    function changed() {   
        var oldStart = field.selectionStart;
        var oldEnd = field.selectionEnd;
        
        field.value = reapplyMask(field.value);
        
        field.selectionStart = oldStart;
        field.selectionEnd = oldEnd;
    }
    
    field.addEventListener('click', changed)
    field.addEventListener('keyup', changed)
}

Date: <input type="text" value="__-__-____" data-mask="__-__-____"/><br/>
Telephone: <input type="text" value="(___) ___-____" data-mask="(___) ___-____"/><br/>

Below i describe my method. I set event on input in input, to call Masking() method, which will return an formatted string of that we insert in input.

Html:

<input name="phone" pattern="+373 __ ___ ___" class="masked" required>

JQ: Here we set event on input:

$('.masked').on('input', function () {
    var input = $(this);
    input.val(Masking(input.val(), input.attr('pattern')));
});

JS: Function, which will format string by pattern;

function Masking (value, pattern) {
var out = '';
var space = ' ';
var any = '_';

for (var i = 0, j = 0; j < value.length; i++, j++) {
    if (value[j] === pattern[i]) {
        out += value[j];
    }
    else if(pattern[i] === any && value[j] !== space) {
        out += value[j];
    }
    else if(pattern[i] === space && value[j] !== space) {
        out += space;
        j--;
    }
    else if(pattern[i] !== any && pattern[i] !== space) {
        out += pattern[i];
        j--;
    }
}

return out;
}

I taken from this thread decision https://stackoverflow.com/questions/12578507/implement-an-input-with-a-mask/55010378#55010378 and adapted it for IE10, and added setter- and getter- functions.

BUT I TESTED FOR PHONE-mask ONLY

$(document).ready(function(){
    var el_arr = document.querySelectorAll("[placeholder][data-slots]");
    for (var el_ind=0; el_ind < el_arr.length; el_ind++ ){
        var el = el_arr[el_ind];
        var pattern = el.getAttribute("placeholder"),
            slots = new Set(el.getAttribute("data-slots") || "_"),
            prev = function(j){return Array.from(pattern, function(c,i){ return slots.has(c)? j=i+1: j;});}(0),
            first = pattern.split('').findIndex(function(c){return slots.has(c);} ),
            accept = new RegExp(el.getAttribute("data-accept") || "\\d", "g"),
            clean = function(input){input = input.match(accept) || [];return Array.from(pattern, function(c){return input[0] === c || slots.has(c) ? input.shift() || c : c;});},
            format = function(){
                var elem = this;
                var i_j_arr = [el.selectionStart, el.selectionEnd].map(function(i){
                    i = clean(el.value.slice(0, i)).findIndex(function(c){ return slots.has(c);});
                    return i<0? prev[prev.length-1]: elem.back? prev[i-1] || first: i;
                });
                el.value = clean(el.value).join('');
                el.setSelectionRange(i_j_arr[0], i_j_arr[1]);
                this.back = false;
            },
            // sdo added
            get_masked_value = function(){
                var input = this.value;
                var ret=[];
                for(var k in pattern){
                    if ( !input[k] )break;
                    if( slots.has(pattern[k]) && input[k]!=pattern[k]){
                        ret.push(input[k]);
                    } 
                } 
                return ret.join('');
            },
            set_masked_value = function(input){
                var ret=[];
                var index_in_value = 0;
                for(var k in pattern){
                    if( slots.has(pattern[k]) && input[index_in_value]){
                        ret.push(input[index_in_value]);
                        index_in_value++;
                    }
                    else{
                        ret.push(pattern[k]);
                    }
                } 
                this.value = ret.join('');
            }                    
        ;
        el.get_masked_value = get_masked_value;
        el.set_masked_value = set_masked_value;
        el.back = false;
        el.addEventListener("keydown", function(event){ this.back = event.key === "Backspace";});
        el.addEventListener("input", format);
        el.addEventListener("focus", format);
        el.addEventListener("blur", function() { return el.value === pattern && (el.value=""); });
    }

});