POSIXextended

POSIX Extended Regular Expressions, or POSIX EREs, are a formal variant of regular expressions defined by the POSIX standard. They are designed to provide a portable, consistent set of pattern-matching capabilities for text processing on POSIX-compliant systems. POSIX EREs are one of two regular-expression flavors described by POSIX, the other being Basic Regular Expressions (BRE).

In POSIX EREs, several metacharacters have special meaning without escaping. Core features include alternation with |, grouping with parentheses, and repetition using +, ?, and {m,n} quantifiers. In addition to these, the usual constructs such as concatenation, the dot wildcard, and character classes (e.g., [a-z]) are supported. This combination allows more expressive patterns than BRE while remaining portable across POSIX tools.

Implementations and usage: The POSIX standard defines interfaces for regular expressions via regcomp and regexec, with the REG_EXTENDED flag enabling ERE syntax. Commands and libraries that rely on POSIX ERE include grep -E, egrep (often equivalent to grep -E), and many programming language libraries that implement REG_EXTENDED. In practice, POSIX EREs are widely used in Unix-like environments to perform complex searches without resorting to non-POSIX engines.

Limitations and context: POSIX EREs do not universally support certain features found in more advanced regex engines, such as backreferences or lookaround assertions. For patterns requiring these capabilities, users commonly rely on PCRE or other non-POSIX engines. Nevertheless, POSIX EREs provide a stable, portable foundation for regex-based text processing across many systems.