In HPFS, file names can be up to 255 characters long (one must be a terminating NULL, "\0"). Directory names can also be 255 characters long, but the length of the complete path, including drive, directories, and file name, cannot exceed 260 characters.

Certain characters that are illegal in the FAT file system are legal in HPFS file names:

    +   =   ;   ,   [   ]

Also, blank spaces can be used anywhere in an HPFS file name or directory name, but blank spaces and periods at the end of a file name are ignored. Additionally, the period (.) is a valid file name character and can be used as many times as desired. There is no requirement that HPFS file names have extensions; however, many applications still create and use them.

An HPFS file name can be all uppercase, all lowercase, or mixed case. The case is preserved for directory listings but is ignored in file searches and all other system operations. Therefore, in a given directory, there cannot be more than one file with the same name when the only difference is case.

File-Naming Rules for Installable File Systems
For file objects managed by OS/2 installable file systems, the following rules apply:

• Each element of a full path name residing on a disk managed by an installable file system can consist of up to 255 characters. File names can be up to 255 characters long (one of the characters must be a terminating NULL, "\0"). Directory names can also be 255 characters long, but the length of the complete path, including drive, directories, and file name, cannot exceed 260 characters. For example, in the path name "c:\XXX...XXX\YYY", "XXX...XXX" can include up to 255 characters. This is referred to as long file names.

• Names are not case sensitive.

• File name case as specified at create time is preserved. This means that if the file "file.TXT" is created, DosFindFirst returns "file.TXT". File name case may be modified using DosMove.

• Blanks immediately preceding a dot are significant. This means that "FILE.TXT" and "FILE .TXT" refer to different files.

• Trailing blanks are truncated. This means that "FILE.TXT " is the same as "FILE.TXT".

• Blanks elsewhere in the name are significant. This means that "F I L E.TXT" is not the same as "FILE.TXT".

• For compatibility reasons, trailing dots on component names are discarded. For Example, "\FILE.TXT...TEXT...\A..B...\C." becomes "\FILE.TXT...TEXT\A..B\C". This processing includes semaphore, queue, pipe, module, shared memory names, and device names.

• The set of legal characters is expanded to include

      +   =   ;   ,   [   ]
  

  as well as all characters legal for the FAT file system.

• If an installable file system uses a component separator within a file name, it must be a dot (.). There are no restrictions on the number of components which can be allowed within a file name, for example "My.Programming.Reference.Part.One".