About UNIX(7)

To answer your question of "UNIX(7)", this is standard convention of saying "The man page for UNIX in section 7 (miscellaneous)".

To open a page like init(8), you would use this command: man 8 init.

man regex vs man 7 regex

To answer your question of the 'subtle differences' between man regex and man 7 regex:

man by default opens the page with the lowest section number. In this case, it is section 3 (Library calls). Opening the page from section 7 (Miscellaneous) is quite different.

Pages in different sections address different things, though in this case they are similar, consider apt:

man -f apt:

apt (8)              - Advanced Package Tool

apt (1)              - annotation processing tool

In this example, apt from section 8 (System administration commands) is completely unrelated to the page from section 1 (Executable programs or shell commands).

For future reference, use man -f PAGE_NAME to locate all pages with that name in all sections. You can then open a page from the section you want with man SECTION PAGE_NAME.

Manpage sections

The various page sections are as follows and can be found from man man:

1. Executable programs or shell commands


2. System calls (functions provided by the kernel)


3. Library calls (functions within program libraries)


4. Special files (usually found in /dev)


5. File formats and conventions eg /etc/passwd


6. Games


7. Miscellaneous (including macro packages and conventions), e.g. man(7), groff(7)


8. System administration commands (usually only for root)


9. Kernel routines [Non standard]