The source code

    /*!
     * Ext JS Library 3.4.0
     * Copyright(c) 2006-2011 Sencha Inc.
     * licensing@sencha.com
     * http://www.sencha.com/license
     */
    /**
     * @class Ext.form.NumberField
     * @extends Ext.form.TextField
     * Numeric text field that provides automatic keystroke filtering and numeric validation.
     * @constructor
     * Creates a new NumberField
     * @param {Object} config Configuration options
     * @xtype numberfield
     */
    Ext.form.NumberField = Ext.extend(Ext.form.TextField,  {
        /**
         * @cfg {RegExp} stripCharsRe @hide
         */
        /**
         * @cfg {RegExp} maskRe @hide
         */
        /**
         * @cfg {String} fieldClass The default CSS class for the field (defaults to "x-form-field x-form-num-field")
         */
        fieldClass: "x-form-field x-form-num-field",

        /**
         * @cfg {Boolean} allowDecimals False to disallow decimal values (defaults to true)
         */
        allowDecimals : true,

        /**
         * @cfg {String} decimalSeparator Character(s) to allow as the decimal separator (defaults to '.')
         */
        decimalSeparator : ".",

        /**
         * @cfg {Number} decimalPrecision The maximum precision to display after the decimal separator (defaults to 2)
         */
        decimalPrecision : 2,

        /**
         * @cfg {Boolean} allowNegative False to prevent entering a negative sign (defaults to true)
         */
        allowNegative : true,

        /**
         * @cfg {Number} minValue The minimum allowed value (defaults to Number.NEGATIVE_INFINITY)
         */
        minValue : Number.NEGATIVE_INFINITY,

        /**
         * @cfg {Number} maxValue The maximum allowed value (defaults to Number.MAX_VALUE)
         */
        maxValue : Number.MAX_VALUE,

        /**
         * @cfg {String} minText Error text to display if the minimum value validation fails (defaults to "The minimum value for this field is {minValue}")
         */
        minText : "The minimum value for this field is {0}",

        /**
         * @cfg {String} maxText Error text to display if the maximum value validation fails (defaults to "The maximum value for this field is {maxValue}")
         */
        maxText : "The maximum value for this field is {0}",

        /**
         * @cfg {String} nanText Error text to display if the value is not a valid number.  For example, this can happen
         * if a valid character like '.' or '-' is left in the field with no number (defaults to "{value} is not a valid number")
         */
        nanText : "{0} is not a valid number",

        /**
         * @cfg {String} baseChars The base set of characters to evaluate as valid numbers (defaults to '0123456789').
         */
        baseChars : "0123456789",

        /**
         * @cfg {Boolean} autoStripChars True to automatically strip not allowed characters from the field. Defaults to false
         */
        autoStripChars: false,

        // private
        initEvents : function() {
            var allowed = this.baseChars + '';
            if (this.allowDecimals) {
                allowed += this.decimalSeparator;
            }
            if (this.allowNegative) {
                allowed += '-';
            }
            allowed = Ext.escapeRe(allowed);
            this.maskRe = new RegExp('[' + allowed + ']');
            if (this.autoStripChars) {
                this.stripCharsRe = new RegExp('[^' + allowed + ']', 'gi');
            }

            Ext.form.NumberField.superclass.initEvents.call(this);
        },

        /**
         * Runs all of NumberFields validations and returns an array of any errors. Note that this first
         * runs TextField's validations, so the returned array is an amalgamation of all field errors.
         * The additional validations run test that the value is a number, and that it is within the
         * configured min and max values.
         * @param {Mixed} value The value to get errors for (defaults to the current field value)
         * @return {Array} All validation errors for this field
         */
        getErrors: function(value) {
            var errors = Ext.form.NumberField.superclass.getErrors.apply(this, arguments);

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

            if (value.length < 1) { // if it's blank and textfield didn't flag it then it's valid
                 return errors;
            }

            value = String(value).replace(this.decimalSeparator, ".");

            if(isNaN(value)){
                errors.push(String.format(this.nanText, value));
            }

            var num = this.parseValue(value);

            if (num < this.minValue) {
                errors.push(String.format(this.minText, this.minValue));
            }

            if (num > this.maxValue) {
                errors.push(String.format(this.maxText, this.maxValue));
            }

            return errors;
        },

        getValue : function() {
            return this.fixPrecision(this.parseValue(Ext.form.NumberField.superclass.getValue.call(this)));
        },

        setValue : function(v) {
        	v = Ext.isNumber(v) ? v : parseFloat(String(v).replace(this.decimalSeparator, "."));
            v = this.fixPrecision(v);
            v = isNaN(v) ? '' : String(v).replace(".", this.decimalSeparator);
            return Ext.form.NumberField.superclass.setValue.call(this, v);
        },

        /**
         * Replaces any existing {@link #minValue} with the new value.
         * @param {Number} value The minimum value
         */
        setMinValue : function(value) {
            this.minValue = Ext.num(value, Number.NEGATIVE_INFINITY);
        },

        /**
         * Replaces any existing {@link #maxValue} with the new value.
         * @param {Number} value The maximum value
         */
        setMaxValue : function(value) {
            this.maxValue = Ext.num(value, Number.MAX_VALUE);
        },

        // private
        parseValue : function(value) {
            value = parseFloat(String(value).replace(this.decimalSeparator, "."));
            return isNaN(value) ? '' : value;
        },

        /**
         * @private
         *
         */
        fixPrecision : function(value) {
            var nan = isNaN(value);

            if (!this.allowDecimals || this.decimalPrecision == -1 || nan || !value) {
                return nan ? '' : value;
            }

            return parseFloat(parseFloat(value).toFixed(this.decimalPrecision));
        },

        beforeBlur : function() {
            var v = this.parseValue(this.getRawValue());

            if (!Ext.isEmpty(v)) {
                this.setValue(v);
            }
        }
    });

    Ext.reg('numberfield', Ext.form.NumberField);