removes all characters from text except for alphanumeric characters (letters and digits) or another set of characters that you specify. This function takes one or two parameters. The first parameter is the source text value to be filtered. The second (optional) parameter is a text string defining the set of characters that will be kept in the source text. If the second parameter is not given, letters and digits will be kept by default.
Conceptually, this is the inverse of the trimAll function, which will remove a set of characters