How do I check if a given string is a legal/valid file name under Windows?

Question

I want to include a batch file rename functionality in my application. A user can type a destination filename pattern and (after replacing some wildcards in the pattern) I need to check if it's going to be a legal filename under Windows. I've tried to use regular expression like [a-zA-Z0-9_]+ but it doesn't include many national-specific characters from various languages (e.g. umlauts and so on). What is the best way to do such a check?

Answer 1

From MSDN's "Naming a File or Directory," here are the general conventions for what a legal file name is under Windows:

You may use any character in the current code page (Unicode/ANSI above 127), except:

• < > : " / \ | ? *
• Characters whose integer representations are 0-31 (less than ASCII space)
• Any other character that the target file system does not allow (say, trailing periods or spaces)
• Any of the DOS names: CON, PRN, AUX, NUL, COM0, COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9, LPT0, LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, LPT9 (and avoid AUX.txt, etc)
• The file name is all periods

Some optional things to check:

• File paths (including the file name) may not have more than 260 characters (that don't use the \?\ prefix)
• Unicode file paths (including the file name) with more than 32,000 characters when using \?\ (note that prefix may expand directory components and cause it to overflow the 32,000 limit)