In computer science, an illegal character is a character that is not allowed by a certain programming language, protocol, or program. To avoid illegal characters, some languages may use an escape character which is a backslash followed by another character.
Examples
Windows In the Windows operating system, illegal characters in file and folder names include colons, brackets, question marks, and null characters. +Illegal characters in Windows file namesCharacterName<less than>greater than:colon"speech marks/forward slash\backslash|pipe?question mark*asterisk
References