POSIX is the acronym for “Portable Operating System Interface for uniX”. It is a collection of standards. This collection define some of the functionality that a (UNIX) operating system should support.
BRE stands for Basic Regular Expressions or BRE, and standardize a flavor similar to the one used by the traditional UNIX grep command.
A BRE supports POSIX bracket expressions, which are similar to character classes in other regex flavors, with a few special features.
In regex a series of shorthand character classes are available. \d is short for [0-9]. In POSIX BRE shorthands are not supported.
The BRE a{1,2} matches a{1,2} literally, while a{1,2} matches a or aa.
Some implementations support \? and + as an alternative syntax to {0,1} and {1,}, but \? and + are not part of the POSIX standard.
Tokens can be grouped with ( and ).
Backreferences are the usual \1 through \9. Only up to 9 groups are permitted. E.g. (ab)\1 matches abab, while (ab)\1 is invalid since there’s no capturing group corresponding to the backreference \1. Use \1 to match \1 literally.
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