No need of regex here. You can use classList
Using classList is a convenient alternative to accessing an element's list of classes as a space-delimited string via element.className.
function ShowHideDtls(itId) {
var subMen = document.getElementById(itId);
if (subMen != null) {
subMen.classList.toggle('in');
}
}
toggle() will toggle the class of the element. If the element already has the class, it'll remove it, if not then toggle will add the class to the element.
Check the Browser Compatibility.
You can use following SHIM from MDN for IE9,
/*
* classList.js: Cross-browser full element.classList implementation.
* 2014-07-23
*
* By Eli Grey, http://eligrey.com
* Public Domain.
* NO WARRANTY EXPRESSED OR IMPLIED. USE AT YOUR OWN RISK.
*/
/*global self, document, DOMException */
/*! @source http://purl.eligrey.com/github/classList.js/blob/master/classList.js*/
if ("document" in self) {
// Full polyfill for browsers with no classList support
if (!("classList" in document.createElement("_"))) {
(function (view) {
"use strict";
if (!('Element' in view)) return;
var
classListProp = "classList",
protoProp = "prototype",
elemCtrProto = view.Element[protoProp],
objCtr = Object,
strTrim = String[protoProp].trim || function () {
return this.replace(/^\s+|\s+$/g, "");
},
arrIndexOf = Array[protoProp].indexOf || function (item) {
var
i = 0,
len = this.length;
for (; i < len; i++) {
if (i in this && this[i] === item) {
return i;
}
}
return -1;
}
// Vendors: please allow content code to instantiate DOMExceptions
,
DOMEx = function (type, message) {
this.name = type;
this.code = DOMException[type];
this.message = message;
},
checkTokenAndGetIndex = function (classList, token) {
if (token === "") {
throw new DOMEx(
"SYNTAX_ERR", "An invalid or illegal string was specified"
);
}
if (/\s/.test(token)) {
throw new DOMEx(
"INVALID_CHARACTER_ERR", "String contains an invalid character"
);
}
return arrIndexOf.call(classList, token);
},
ClassList = function (elem) {
var
trimmedClasses = strTrim.call(elem.getAttribute("class") || ""),
classes = trimmedClasses ? trimmedClasses.split(/\s+/) : [],
i = 0,
len = classes.length;
for (; i < len; i++) {
this.push(classes[i]);
}
this._updateClassName = function () {
elem.setAttribute("class", this.toString());
};
},
classListProto = ClassList[protoProp] = [],
classListGetter = function () {
return new ClassList(this);
};
// Most DOMException implementations don't allow calling DOMException's toString()
// on non-DOMExceptions. Error's toString() is sufficient here.
DOMEx[protoProp] = Error[protoProp];
classListProto.item = function (i) {
return this[i] || null;
};
classListProto.contains = function (token) {
token += "";
return checkTokenAndGetIndex(this, token) !== -1;
};
classListProto.add = function () {
var
tokens = arguments,
i = 0,
l = tokens.length,
token, updated = false;
do {
token = tokens[i] + "";
if (checkTokenAndGetIndex(this, token) === -1) {
this.push(token);
updated = true;
}
}
while (++i < l);
if (updated) {
this._updateClassName();
}
};
classListProto.remove = function () {
var
tokens = arguments,
i = 0,
l = tokens.length,
token, updated = false,
index;
do {
token = tokens[i] + "";
index = checkTokenAndGetIndex(this, token);
while (index !== -1) {
this.splice(index, 1);
updated = true;
index = checkTokenAndGetIndex(this, token);
}
}
while (++i < l);
if (updated) {
this._updateClassName();
}
};
classListProto.toggle = function (token, force) {
token += "";
var
result = this.contains(token),
method = result ?
force !== true && "remove" :
force !== false && "add";
if (method) {
this[method](token);
}
if (force === true || force === false) {
return force;
} else {
return !result;
}
};
classListProto.toString = function () {
return this.join(" ");
};
if (objCtr.defineProperty) {
var classListPropDesc = {
get: classListGetter,
enumerable: true,
configurable: true
};
try {
objCtr.defineProperty(elemCtrProto, classListProp, classListPropDesc);
} catch (ex) { // IE 8 doesn't support enumerable:true
if (ex.number === -0x7FF5EC54) {
classListPropDesc.enumerable = false;
objCtr.defineProperty(elemCtrProto, classListProp, classListPropDesc);
}
}
} else if (objCtr[protoProp].__defineGetter__) {
elemCtrProto.__defineGetter__(classListProp, classListGetter);
}
}(self));
} else {
// There is full or partial native classList support, so just check if we need
// to normalize the add/remove and toggle APIs.
(function () {
"use strict";
var testElement = document.createElement("_");
testElement.classList.add("c1", "c2");
// Polyfill for IE 10/11 and Firefox <26, where classList.add and
// classList.remove exist but support only one argument at a time.
if (!testElement.classList.contains("c2")) {
var createMethod = function (method) {
var original = DOMTokenList.prototype[method];
DOMTokenList.prototype[method] = function (token) {
var i, len = arguments.length;
for (i = 0; i < len; i++) {
token = arguments[i];
original.call(this, token);
}
};
};
createMethod('add');
createMethod('remove');
}
testElement.classList.toggle("c3", false);
// Polyfill for IE 10 and Firefox <24, where classList.toggle does not
// support the second argument.
if (testElement.classList.contains("c3")) {
var _toggle = DOMTokenList.prototype.toggle;
DOMTokenList.prototype.toggle = function (token, force) {
if (1 in arguments && !this.contains(token) === !force) {
return force;
} else {
return _toggle.call(this, token);
}
};
}
testElement = null;
}());
}
}
If you're using jQuery, you can use toggleClass():
function ShowHideDtls(itId) {
$('#' + itId).toggleClass('in');
}
Edit
If you still want to use regex:
if (/\bin\b/.test(subMen.className))
subMen.className.replace(/\bin\b/, '');
} else {
subMen.className += " in";
}
You can also use split() and indexOf as follow to check if a class is present on element.
var classes = className.split(/\s+/),
classIndex = classes.indexOf('in');
if (classIndex > -1) {
classes.splice(classIndex, 1);
subMen.className = classes.join(' ');
} else {
subMen.className += " in";
}
