var strReplaceAll = strText;
var intIndexOfMatch = strReplaceAll.indexOf( "th" );
 
// Loop over the string value replacing out each matching
// substring.
while (intIndexOfMatch != -1){
	// Relace out the current instance.
	strReplaceAll = strReplaceAll.replace( "th", "[X]" )
	 
	// Get the index of any next matching substring.
	intIndexOfMatch = strReplaceAll.indexOf( "th" );
}
 
alert( strReplaceAll );