In the 1980s, with the various Unix vendors such as IBM, HP, Digital and SunMicrosystems each adding their own features, unix was in danger of becoming fragmented, making it harder to write portable programs. (Also the "BSD vs. System V" split.)

POSIX is a large (some might even say unwieldy) standard (officially IEEE Standard 1003) specifying things such as the behaviour (arguments, etc) of standard programs and utilities that must exist on a system and C library functions. The theory is that if you write a program that conforms to POSIX standards, it will compile and run on any POSIX-compliant operating system.

1003.1 defines the core functions and behaviours, as well as things like the standard ErrorMessages.

1003.2 defines standard RegularExpressions? syntax and behaviour