Replacing illegal character in fileName

You need to replace everything but [a-zA-Z0-9.-]. The ^ within the brackets stands for "NOT".

myString = myString.replaceAll("[^a-zA-Z0-9\\.\\-]", "_");

If you are looking for options on windows platform then you can try below solution to make use of all valid characters other than "\/:*?"<>|" in file name.

fileName = fileName.replaceAll("[\\\\/:*?\"<>|]", "_");