The source code

    /**
     * @class Ext.form.TextField
     * @extends Ext.form.Field
     * Basic text field.  Can be used as a direct replacement for traditional text inputs,
     * or as the base class for more sophisticated input controls (like {@link Ext.form.TextArea}
     * and {@link Ext.form.ComboBox}).
     * Validation
     * Field validation is processed in a particular order.  If validation fails at any particular
     * step the validation routine halts.
     *
     * 1. Field specific validator
     *
     * If a field is configured with a {@link Ext.form.TextField#validator validator} function,
     * it will be passed the current field value.  The {@link Ext.form.TextField#validator validator}
     * function is expected to return boolean true if the value is valid or return a string to
     * represent the invalid message if invalid.
     *
     * 2. Built in Validation
     *
     * Basic validation is affected with the following configuration properties:
     *
     * Validation    Invalid Message
     * {@link Ext.form.TextField#allowBlank allowBlank}    {@link Ext.form.TextField#emptyText emptyText}
     * {@link Ext.form.TextField#minLength minLength}     {@link Ext.form.TextField#minLengthText minLengthText}
     * {@link Ext.form.TextField#maxLength maxLength}     {@link Ext.form.TextField#maxLengthText maxLengthText}
     *
     *
     * 3. Preconfigured Validation Types (VTypes)
     *
     * Using VTypes offers a convenient way to reuse validation. If a field is configured with a
     * {@link Ext.form.TextField#vtype vtype}, the corresponding {@link Ext.form.VTypes VTypes}
     * validation function will be used for validation.  If invalid, either the field's
     * {@link Ext.form.TextField#vtypeText vtypeText} or the VTypes vtype Text property will be
     * used for the invalid message.  Keystrokes on the field will be filtered according to the VTypes
     * vtype Mask property.
     *
     * 4. Field specific regex test
     *
     * Each field may also specify a {@link Ext.form.TextField#regex regex} test.
     * The invalid message for this test is configured with
     * {@link Ext.form.TextField#regexText regexText}.
     *
     * Alter Validation Behavior
     *
     * Validation behavior for each field can be configured:
     * {@link Ext.form.TextField#invalidText invalidText} : the default validation message to
     * show if any validation step above does not provide a message when invalid
     * {@link Ext.form.TextField#maskRe maskRe} : filter out keystrokes before any validation occurs
     * {@link Ext.form.TextField#stripCharsRe stripCharsRe} : filter characters after being typed in,
     * but before being validated
     * {@link Ext.form.Field#invalidClass invalidClass} : alternate style when invalid
     * {@link Ext.form.Field#validateOnBlur validateOnBlur},
     * {@link Ext.form.Field#validationDelay validationDelay}, and
     * {@link Ext.form.Field#validationEvent validationEvent} : modify how/when validation is triggered
     *
     *
     *
     * @constructor
     * Creates a new TextField
     * @param {Object} config Configuration options
     * @xtype textfield
     */
    Ext.form.TextField = Ext.extend(Ext.form.Field,  {
        /**
         * @cfg {String} vtypeText A custom error message to display in place of the default message provided
         * for the {@link #vtype} currently set for this field (defaults to '').  Note:
         * only applies if {@link #vtype} is set, else ignored.
         */
        /**
         * @cfg {RegExp} stripCharsRe A JavaScript RegExp object used to strip unwanted content from the value
         * before validation (defaults to null).
         */
        /**
         * @cfg {Boolean} grow true if this field should automatically grow and shrink to its content
         * (defaults to false)
         */
        grow : false,
        /**
         * @cfg {Number} growMin The minimum width to allow when {@link #grow} = true (defaults
         * to 30)
         */
        growMin : 30,
        /**
         * @cfg {Number} growMax The maximum width to allow when {@link #grow} = true (defaults
         * to 800)
         */
        growMax : 800,
        /**
         * @cfg {String} vtype A validation type name as defined in {@link Ext.form.VTypes} (defaults to null)
         */
        vtype : null,
        /**
         * @cfg {RegExp} maskRe An input mask regular expression that will be used to filter keystrokes that do
         * not match (defaults to null)
         */
        maskRe : null,
        /**
         * @cfg {Boolean} disableKeyFilter Specify true to disable input keystroke filtering (defaults
         * to false)
         */
        disableKeyFilter : false,
        /**
         * @cfg {Boolean} allowBlank Specify false to validate that the value's length is > 0 (defaults to
         * true)
         */
        allowBlank : true,
        /**
         * @cfg {Number} minLength Minimum input field length required (defaults to 0)
         */
        minLength : 0,
        /**
         * @cfg {Number} maxLength Maximum input field length allowed by validation (defaults to Number.MAX_VALUE).
         * This behavior is intended to provide instant feedback to the user by improving usability to allow pasting
         * and editing or overtyping and back tracking. To restrict the maximum number of characters that can be
         * entered into the field use {@link Ext.form.Field#autoCreate autoCreate} to add
         * any attributes you want to a field, for example:
    var myField = new Ext.form.NumberField({
        id: 'mobile',
        anchor:'90%',
        fieldLabel: 'Mobile',
        maxLength: 16, // for validation
        autoCreate: {tag: 'input', type: 'text', size: '20', autocomplete: 'off', maxlength: '10'}
    });

         */
        maxLength : Number.MAX_VALUE,
        /**
         * @cfg {String} minLengthText Error text to display if the {@link #minLength minimum length}
         * validation fails (defaults to 'The minimum length for this field is {minLength}')
         */
        minLengthText : 'The minimum length for this field is {0}',
        /**
         * @cfg {String} maxLengthText Error text to display if the {@link #maxLength maximum length}
         * validation fails (defaults to 'The maximum length for this field is {maxLength}')
         */
        maxLengthText : 'The maximum length for this field is {0}',
        /**
         * @cfg {Boolean} selectOnFocus true to automatically select any existing field text when the field
         * receives input focus (defaults to false)
         */
        selectOnFocus : false,
        /**
         * @cfg {String} blankText The error text to display if the {@link #allowBlank} validation
         * fails (defaults to 'This field is required')
         */
        blankText : 'This field is required',
        /**
         * @cfg {Function} validator A custom validation function to be called during field validation
         * (defaults to null). If specified, this function will be called first, allowing the
         * developer to override the default validation process. This function will be passed the current
         * field value and expected to return boolean true if the value is valid or a string
         * error message if invalid.
         */
        validator : null,
        /**
         * @cfg {RegExp} regex A JavaScript RegExp object to be tested against the field value during validation
         * (defaults to null). If the test fails, the field will be marked invalid using
         * {@link #regexText}.
         */
        regex : null,
        /**
         * @cfg {String} regexText The error text to display if {@link #regex} is used and the
         * test fails during validation (defaults to '')
         */
        regexText : '',
        /**
         * @cfg {String} emptyText The default text to place into an empty field (defaults to null).
         * Note: that this value will be submitted to the server if this field is enabled and configured
         * with a {@link #name}.
         */
        emptyText : null,
        /**
         * @cfg {String} emptyClass The CSS class to apply to an empty field to style the {@link #emptyText}
         * (defaults to 'x-form-empty-field').  This class is automatically added and removed as needed
         * depending on the current field value.
         */
        emptyClass : 'x-form-empty-field',

        /**
         * @cfg {Boolean} enableKeyEvents true to enable the proxying of key events for the HTML input
         * field (defaults to false)
         */

        initComponent : function(){
            Ext.form.TextField.superclass.initComponent.call(this);
            this.addEvents(
                /**
                 * @event autosize
                 * Fires when the {@link #autoSize} function is triggered. The field may or
                 * may not have actually changed size according to the default logic, but this event provides
                 * a hook for the developer to apply additional logic at runtime to resize the field if needed.
                 * @param {Ext.form.Field} this This text field
                 * @param {Number} width The new field width
                 */
                'autosize',

                /**
                 * @event keydown
                 * Keydown input field event. This event only fires if {@link #enableKeyEvents}
                 * is set to true.
                 * @param {Ext.form.TextField} this This text field
                 * @param {Ext.EventObject} e
                 */
                'keydown',
                /**
                 * @event keyup
                 * Keyup input field event. This event only fires if {@link #enableKeyEvents}
                 * is set to true.
                 * @param {Ext.form.TextField} this This text field
                 * @param {Ext.EventObject} e
                 */
                'keyup',
                /**
                 * @event keypress
                 * Keypress input field event. This event only fires if {@link #enableKeyEvents}
                 * is set to true.
                 * @param {Ext.form.TextField} this This text field
                 * @param {Ext.EventObject} e
                 */
                'keypress'
            );
        },

        // private
        initEvents : function(){
            Ext.form.TextField.superclass.initEvents.call(this);
            if(this.validationEvent == 'keyup'){
                this.validationTask = new Ext.util.DelayedTask(this.validate, this);
                this.mon(this.el, 'keyup', this.filterValidation, this);
            }
            else if(this.validationEvent !== false){
            	this.mon(this.el, this.validationEvent, this.validate, this, {buffer: this.validationDelay});
            }
            if(this.selectOnFocus || this.emptyText){
                this.on('focus', this.preFocus, this);

                this.mon(this.el, 'mousedown', function(){
                    if(!this.hasFocus){
                        this.el.on('mouseup', function(e){
                            e.preventDefault();
                        }, this, {single:true});
                    }
                }, this);

                if(this.emptyText){
                    this.on('blur', this.postBlur, this);
                    this.applyEmptyText();
                }
            }
            if(this.maskRe || (this.vtype && this.disableKeyFilter !== true && (this.maskRe = Ext.form.VTypes[this.vtype+'Mask']))){
            	this.mon(this.el, 'keypress', this.filterKeys, this);
            }
            if(this.grow){
            	this.mon(this.el, 'keyup', this.onKeyUpBuffered, this, {buffer: 50});
    			this.mon(this.el, 'click', this.autoSize, this);
            }
            if(this.enableKeyEvents){
            	this.mon(this.el, 'keyup', this.onKeyUp, this);
            	this.mon(this.el, 'keydown', this.onKeyDown, this);
            	this.mon(this.el, 'keypress', this.onKeyPress, this);
            }
        },

        processValue : function(value){
            if(this.stripCharsRe){
                var newValue = value.replace(this.stripCharsRe, '');
                if(newValue !== value){
                    this.setRawValue(newValue);
                    return newValue;
                }
            }
            return value;
        },

        filterValidation : function(e){
            if(!e.isNavKeyPress()){
                this.validationTask.delay(this.validationDelay);
            }
        },

        //private
        onDisable: function(){
            Ext.form.TextField.superclass.onDisable.call(this);
            if(Ext.isIE){
                this.el.dom.unselectable = 'on';
            }
        },

        //private
        onEnable: function(){
            Ext.form.TextField.superclass.onEnable.call(this);
            if(Ext.isIE){
                this.el.dom.unselectable = '';
            }
        },

        // private
        onKeyUpBuffered : function(e){
            if(!e.isNavKeyPress()){
                this.autoSize();
            }
        },

        // private
        onKeyUp : function(e){
            this.fireEvent('keyup', this, e);
        },

        // private
        onKeyDown : function(e){
            this.fireEvent('keydown', this, e);
        },

        // private
        onKeyPress : function(e){
            this.fireEvent('keypress', this, e);
        },

        /**
         * Resets the current field value to the originally-loaded value and clears any validation messages.
         * Also adds {@link #emptyText} and {@link #emptyClass} if the
         * original value was blank.
         */
        reset : function(){
            Ext.form.TextField.superclass.reset.call(this);
            this.applyEmptyText();
        },

        applyEmptyText : function(){
            if(this.rendered && this.emptyText && this.getRawValue().length < 1 && !this.hasFocus){
                this.setRawValue(this.emptyText);
                this.el.addClass(this.emptyClass);
            }
        },

        // private
        preFocus : function(){
            var el = this.el;
            if(this.emptyText){
                if(el.dom.value == this.emptyText){
                    this.setRawValue('');
                }
                el.removeClass(this.emptyClass);
            }
            if(this.selectOnFocus){
                (function(){
                    el.dom.select();
                }).defer(this.inEditor && Ext.isIE ? 50 : 0);
            }
        },

        // private
        postBlur : function(){
            this.applyEmptyText();
        },

        // private
        filterKeys : function(e){
            // special keys don't generate charCodes, so leave them alone
            if(e.ctrlKey || e.isSpecialKey()){
                return;
            }

            if(!this.maskRe.test(String.fromCharCode(e.getCharCode()))){
                e.stopEvent();
            }
        },

        setValue : function(v){
            if(this.emptyText && this.el && !Ext.isEmpty(v)){
                this.el.removeClass(this.emptyClass);
            }
            Ext.form.TextField.superclass.setValue.apply(this, arguments);
            this.applyEmptyText();
            this.autoSize();
            return this;
        },

        /**
         * Validates a value according to the field's validation rules and marks the field as invalid
         * if the validation fails
         * @param {Mixed} value The value to validate
         * @return {Boolean} True if the value is valid, else false
         */
        validateValue : function(value){
            if(Ext.isFunction(this.validator)){
                var msg = this.validator(value);
                if(msg !== true){
                    this.markInvalid(msg);
                    return false;
                }
            }
            if(value.length < 1 || value === this.emptyText){ // if it's blank
                 if(this.allowBlank){
                     this.clearInvalid();
                     return true;
                 }else{
                     this.markInvalid(this.blankText);
                     return false;
                 }
            }
            if(value.length < this.minLength){
                this.markInvalid(String.format(this.minLengthText, this.minLength));
                return false;
            }
            if(value.length > this.maxLength){
                this.markInvalid(String.format(this.maxLengthText, this.maxLength));
                return false;
            }
            if(this.vtype){
                var vt = Ext.form.VTypes;
                if(!vt[this.vtype](value, this)){
                    this.markInvalid(this.vtypeText || vt[this.vtype +'Text']);
                    return false;
                }
            }
            if(this.regex && !this.regex.test(value)){
                this.markInvalid(this.regexText);
                return false;
            }
            return true;
        },

        /**
         * Selects text in this field
         * @param {Number} start (optional) The index where the selection should start (defaults to 0)
         * @param {Number} end (optional) The index where the selection should end (defaults to the text length)
         */
        selectText : function(start, end){
            var v = this.getRawValue();
            var doFocus = false;
            if(v.length > 0){
                start = start === undefined ? 0 : start;
                end = end === undefined ? v.length : end;
                var d = this.el.dom;
                if(d.setSelectionRange){
                    d.setSelectionRange(start, end);
                }else if(d.createTextRange){
                    var range = d.createTextRange();
                    range.moveStart('character', start);
                    range.moveEnd('character', end-v.length);
                    range.select();
                }
                doFocus = Ext.isGecko || Ext.isOpera;
            }else{
                doFocus = true;
            }
            if(doFocus){
                this.focus();
            }
        },

        /**
         * Automatically grows the field to accomodate the width of the text up to the maximum field width allowed.
         * This only takes effect if {@link #grow} = true, and fires the {@link #autosize} event.
         */
        autoSize : function(){
            if(!this.grow || !this.rendered){
                return;
            }
            if(!this.metrics){
                this.metrics = Ext.util.TextMetrics.createInstance(this.el);
            }
            var el = this.el;
            var v = el.dom.value;
            var d = document.createElement('div');
            d.appendChild(document.createTextNode(v));
            v = d.innerHTML;
            d = null;
            Ext.removeNode(d);
            v += ' ';
            var w = Math.min(this.growMax, Math.max(this.metrics.getWidth(v) + /* add extra padding */ 10, this.growMin));
            this.el.setWidth(w);
            this.fireEvent('autosize', this, w);
        },

    	onDestroy: function(){
    		if(this.validationTask){
    			this.validationTask.cancel();
    			this.validationTask = null;
    		}
    		Ext.form.TextField.superclass.onDestroy.call(this);
    	}
    });
    Ext.reg('textfield', Ext.form.TextField);