There is a function String.replace() which returns the result string. But note, this only replaces the first occurance of given string/expression!

 

So, to actually replace all the occurances of given string/expression, you must loop through until all the matches are replaced. Code & Documentation

However, there is another easy customized function to automatically replace all the occurance of given match. Here goes the customized function:

public function str_replace(mainStr:String, str1:String, str2:String):String
 {
 var temp:Array = mainStr.split(str1);
 var tempStr:String = temp.join(str2);
 return tempStr;
 }

Usage:

var myStr:String = str_replace("a_b_c", "_" , "1");   // a1b1c1

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