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POSIX Basic Vs. Extended Regular Expressions
Just when we thought this couldn’t get any more confusing, we discover that POSIX also splits regular expression implementations into two kinds: basic regular expressions (BRE) and extended regular expressions (ERE). The features we have covered so far are supported by any application that is POSIX compliant and implements BRE. Our grep program is one such program.
What’s the difference between BRE and ERE? It’s a matter of metacharacters. With BRE, the following metacharacters are recognized:
^ $ . [ ] *
All other characters are considered literals. With ERE, the following metacharacters (and their associated functions) are added:
( ) { } ? + |
However (and this is the fun part), the “(”, “)”, “{”, and “}” characters are treated as metacharacters in BRE if they are escaped with a backslash, whereas with ERE, preced- ing any metacharacter with a backslash causes it to be treated as a literal. Any weirdness that comes along will be covered in the discussions that follow.
Since the features we are going to discuss next are part of ERE, we are going to need to use a different grep. Traditionally, this has been performed by the egrep program, but the GNU version of grep also supports extended regular expressions when the -E op- tion is used.
POSIX
During the 1980’s, Unix became a very popular commercial operating system, but by 1988, the Unix world was in turmoil. Many computer manufacturers had li- censed the Unix source code from its creators, AT&T, and were supplying various versions of the operating system with their systems. However, in their efforts to create product differentiation, each manufacturer added proprietary changes and extensions. This started to limit the compatibility of the software. As always with
POSIX Basic Vs. Extended Regular Expressions
proprietary vendors, each was trying to play a winning game of “lock-in” with their customers. This dark time in the history of Unix is known today as “the Balkanization.”
Enter the IEEE (Institute of Electrical and Electronics Engineers). In the mid- 1980s, the IEEE began developing a set of standards that would define how Unix (and Unix-like) systems would perform. These standards, formally known as IEEE 1003, define the application programming interfaces (APIs), shell and utili- ties that are to be found on a standard Unix-like system. The name “POSIX,” which stands for Portable Operating System Interface (with the “X” added to the end for extra snappiness), was suggested by Richard Stallman (yes, that Richard Stallman), and was adopted by the IEEE.