The source code

    /**
     * @class Ext.form.TextField
     * @extends Ext.form.Field
     * <p>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}).</p>
     * <p><b><u>Validation</u></b></p>
     * <p>The validation procedure is described in the documentation for {@link #validateValue}.</p>
     * <p><b><u>Alter Validation Behavior</u></b></p>
     * <p>Validation behavior for each field can be configured:</p>
     * <div class="mdetail-params"><ul>
     * <li><code>{@link Ext.form.TextField#invalidText invalidText}</code> : the default validation message to
     * show if any validation step above does not provide a message when invalid</li>
     * <li><code>{@link Ext.form.TextField#maskRe maskRe}</code> : filter out keystrokes before any validation occurs</li>
     * <li><code>{@link Ext.form.TextField#stripCharsRe stripCharsRe}</code> : filter characters after being typed in,
     * but before being validated</li>
     * <li><code>{@link Ext.form.Field#invalidClass invalidClass}</code> : alternate style when invalid</li>
     * <li><code>{@link Ext.form.Field#validateOnBlur validateOnBlur}</code>,
     * <code>{@link Ext.form.Field#validationDelay validationDelay}</code>, and
     * <code>{@link Ext.form.Field#validationEvent validationEvent}</code> : modify how/when validation is triggered</li>
     * </ul></div>
     *
     * @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 <b><code>{@link #vtype}</code></b> currently set for this field (defaults to <tt>''</tt>).  <b>Note</b>:
         * only applies if <b><code>{@link #vtype}</code></b> is set, else ignored.
         */
        /**
         * @cfg {RegExp} stripCharsRe A JavaScript RegExp object used to strip unwanted content from the value
         * before validation (defaults to <tt>null</tt>).
         */
        /**
         * @cfg {Boolean} grow <tt>true</tt> if this field should automatically grow and shrink to its content
         * (defaults to <tt>false</tt>)
         */
        grow : false,
        /**
         * @cfg {Number} growMin The minimum width to allow when <code><b>{@link #grow}</b> = true</code> (defaults
         * to <tt>30</tt>)
         */
        growMin : 30,
        /**
         * @cfg {Number} growMax The maximum width to allow when <code><b>{@link #grow}</b> = true</code> (defaults
         * to <tt>800</tt>)
         */
        growMax : 800,
        /**
         * @cfg {String} vtype A validation type name as defined in {@link Ext.form.VTypes} (defaults to <tt>null</tt>)
         */
        vtype : null,
        /**
         * @cfg {RegExp} maskRe An input mask regular expression that will be used to filter keystrokes that do
         * not match (defaults to <tt>null</tt>). The maskRe will not operate on any paste events.
         */
        maskRe : null,
        /**
         * @cfg {Boolean} disableKeyFilter Specify <tt>true</tt> to disable input keystroke filtering (defaults
         * to <tt>false</tt>)
         */
        disableKeyFilter : false,
        /**
         * @cfg {Boolean} allowBlank Specify <tt>false</tt> to validate that the value's length is > 0 (defaults to
         * <tt>true</tt>)
         */
        allowBlank : true,
        /**
         * @cfg {Number} minLength Minimum input field length required (defaults to <tt>0</tt>)
         */
        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 <tt><b>{@link Ext.form.Field#autoCreate autoCreate}</b></tt> to add
         * any attributes you want to a field, for example:<pre><code>
    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'}
    });
    </code></pre>
         */
        maxLength : Number.MAX_VALUE,
        /**
         * @cfg {String} minLengthText Error text to display if the <b><tt>{@link #minLength minimum length}</tt></b>
         * validation fails (defaults to <tt>'The minimum length for this field is {minLength}'</tt>)
         */
        minLengthText : 'The minimum length for this field is {0}',
        /**
         * @cfg {String} maxLengthText Error text to display if the <b><tt>{@link #maxLength maximum length}</tt></b>
         * validation fails (defaults to <tt>'The maximum length for this field is {maxLength}'</tt>)
         */
        maxLengthText : 'The maximum length for this field is {0}',
        /**
         * @cfg {Boolean} selectOnFocus <tt>true</tt> to automatically select any existing field text when the field
         * receives input focus (defaults to <tt>false</tt>)
         */
        selectOnFocus : false,
        /**
         * @cfg {String} blankText The error text to display if the <b><tt>{@link #allowBlank}</tt></b> validation
         * fails (defaults to <tt>'This field is required'</tt>)
         */
        blankText : 'This field is required',
        /**
         * @cfg {Function} validator
         * <p>A custom validation function to be called during field validation ({@link #validateValue})
         * (defaults to <tt>null</tt>). If specified, this function will be called first, allowing the
         * developer to override the default validation process.</p>
         * <br><p>This function will be passed the following Parameters:</p>
         * <div class="mdetail-params"><ul>
         * <li><code>value</code>: <i>Mixed</i>
         * <div class="sub-desc">The current field value</div></li>
         * </ul></div>
         * <br><p>This function is to Return:</p>
         * <div class="mdetail-params"><ul>
         * <li><code>true</code>: <i>Boolean</i>
         * <div class="sub-desc"><code>true</code> if the value is valid</div></li>
         * <li><code>msg</code>: <i>String</i>
         * <div class="sub-desc">An error message if the value is invalid</div></li>
         * </ul></div>
         */
        validator : null,
        /**
         * @cfg {RegExp} regex A JavaScript RegExp object to be tested against the field value during validation
         * (defaults to <tt>null</tt>). If the test fails, the field will be marked invalid using
         * <b><tt>{@link #regexText}</tt></b>.
         */
        regex : null,
        /**
         * @cfg {String} regexText The error text to display if <b><tt>{@link #regex}</tt></b> is used and the
         * test fails during validation (defaults to <tt>''</tt>)
         */
        regexText : '',
        /**
         * @cfg {String} emptyText The default text to place into an empty field (defaults to <tt>null</tt>).
         * <b>Note</b>: 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 <b><tt>{@link #emptyText}</tt></b>
         * (defaults to <tt>'x-form-empty-field'</tt>).  This class is automatically added and removed as needed
         * depending on the current field value.
         */
        emptyClass : 'x-form-empty-field',

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

        initComponent : function(){
            Ext.form.TextField.superclass.initComponent.call(this);
            this.addEvents(
                /**
                 * @event autosize
                 * Fires when the <tt><b>{@link #autoSize}</b></tt> 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 <tt><b>{@link #enableKeyEvents}</b></tt>
                 * 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 <tt><b>{@link #enableKeyEvents}</b></tt>
                 * 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 <tt><b>{@link #enableKeyEvents}</b></tt>
                 * 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.validationEvent != 'blur'){
            	this.mon(this.el, this.validationEvent, this.validate, this, {buffer: this.validationDelay});
            }
            if(this.selectOnFocus || this.emptyText){
                this.mon(this.el, 'mousedown', this.onMouseDown, this);

                if(this.emptyText){
                    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, {
                    scope: this,
                    keyup: this.onKeyUp,
                    keydown: this.onKeyDown,
                    keypress: this.onKeyPress
                });
            }
        },

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

        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(this.doAutoSize(e)){
                this.autoSize();
            }
        },

        // private
        doAutoSize : function(e){
            return !e.isNavKeyPress();
        },

        // 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 <tt><b>{@link #emptyText}</b></tt> and <tt><b>{@link #emptyClass}</b></tt> 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,
                isEmpty;
            if(this.emptyText){
                if(el.dom.value == this.emptyText){
                    this.setRawValue('');
                    isEmpty = true;
                }
                el.removeClass(this.emptyClass);
            }
            if(this.selectOnFocus || isEmpty){
                el.dom.select();
            }
        },

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

        // private
        filterKeys : function(e){
            if(e.ctrlKey){
                return;
            }
            var k = e.getKey();
            if(Ext.isGecko && (e.isNavKeyPress() || k == e.BACKSPACE || (k == e.DELETE && e.button == -1))){
                return;
            }
            var cc = String.fromCharCode(e.getCharCode());
            if(!Ext.isGecko && e.isSpecialKey() && !cc){
                return;
            }
            if(!this.maskRe.test(cc)){
                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;
        },

        /**
         * <p>Validates a value according to the field's validation rules and returns an array of errors
         * for any failing validations. Validation rules are processed in the following order:</p>
         * <div class="mdetail-params"><ul>
         *
         * <li><b>1. Field specific validator</b>
         * <div class="sub-desc">
         * <p>A validator offers a way to customize and reuse a validation specification.
         * If a field is configured with a <code>{@link #validator}</code>
         * function, it will be passed the current field value.  The <code>{@link #validator}</code>
         * function is expected to return either:
         * <div class="mdetail-params"><ul>
         * <li>Boolean <tt>true</tt> if the value is valid (validation continues).</li>
         * <li>a String to represent the invalid message if invalid (validation halts).</li>
         * </ul></div>
         * </div></li>
         *
         * <li><b>2. Basic Validation</b>
         * <div class="sub-desc">
         * <p>If the <code>{@link #validator}</code> has not halted validation,
         * basic validation proceeds as follows:</p>
         *
         * <div class="mdetail-params"><ul>
         *
         * <li><code>{@link #allowBlank}</code> : (Invalid message =
         * <code>{@link #emptyText}</code>)<div class="sub-desc">
         * Depending on the configuration of <code>{@link #allowBlank}</code>, a
         * blank field will cause validation to halt at this step and return
         * Boolean true or false accordingly.
         * </div></li>
         *
         * <li><code>{@link #minLength}</code> : (Invalid message =
         * <code>{@link #minLengthText}</code>)<div class="sub-desc">
         * If the passed value does not satisfy the <code>{@link #minLength}</code>
         * specified, validation halts.
         * </div></li>
         *
         * <li><code>{@link #maxLength}</code> : (Invalid message =
         * <code>{@link #maxLengthText}</code>)<div class="sub-desc">
         * If the passed value does not satisfy the <code>{@link #maxLength}</code>
         * specified, validation halts.
         * </div></li>
         *
         * </ul></div>
         * </div></li>
         *
         * <li><b>3. Preconfigured Validation Types (VTypes)</b>
         * <div class="sub-desc">
         * <p>If none of the prior validation steps halts validation, a field
         * configured with a <code>{@link #vtype}</code> will utilize the
         * corresponding {@link Ext.form.VTypes VTypes} validation function.
         * If invalid, either the field's <code>{@link #vtypeText}</code> 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.</p>
         * </div></li>
         *
         * <li><b>4. Field specific regex test</b>
         * <div class="sub-desc">
         * <p>If none of the prior validation steps halts validation, a field's
         * configured <code>{@link #regex}</code> test will be processed.
         * The invalid message for this test is configured with
         * <code>{@link #regexText}</code>.</p>
         * </div></li>
         *
         * @param {Mixed} value The value to validate. The processed raw value will be used if nothing is passed
         * @return {Array} Array of any validation errors
         */
        getErrors: function(value) {
            var errors = Ext.form.TextField.superclass.getErrors.apply(this, arguments);

            value = Ext.isDefined(value) ? value : this.processValue(this.getRawValue());

            if (Ext.isFunction(this.validator)) {
                var msg = this.validator(value);
                if (msg !== true) {
                    errors.push(msg);
                }
            }

            if (value.length < 1 || value === this.emptyText) {
                if (this.allowBlank) {
                    //if value is blank and allowBlank is true, there cannot be any additional errors
                    return errors;
                } else {
                    errors.push(this.blankText);
                }
            }

            if (!this.allowBlank && (value.length < 1 || value === this.emptyText)) { // if it's blank
                errors.push(this.blankText);
            }

            if (value.length < this.minLength) {
                errors.push(String.format(this.minLengthText, this.minLength));
            }

            if (value.length > this.maxLength) {
                errors.push(String.format(this.maxLengthText, this.maxLength));
            }

            if (this.vtype) {
                var vt = Ext.form.VTypes;
                if(!vt[this.vtype](value, this)){
                    errors.push(this.vtypeText || vt[this.vtype +'Text']);
                }
            }

            if (this.regex && !this.regex.test(value)) {
                errors.push(this.regexText);
            }

            return errors;
        },

        /**
         * 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 <tt><b>{@link #grow}</b> = true</tt>, 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;
            Ext.removeNode(d);
            d = null;
            v += '&#160;';
            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);