egrep

egrep is a Unix command-line utility that searches text for lines matching a pattern using extended regular expressions (ERE). It originated as part of the grep family and is traditionally described as an enhanced version of grep that supports richer pattern syntax. The name stands for extended grep. On many modern systems, egrep is implemented as a wrapper around grep with the -E option, or as a symbolic link to grep, making its behavior equivalent to grep -E.

Extended regular expressions permit constructs such as alternation with the vertical bar and grouping with parentheses, as well as other metacharacters supported by the engine. Because egrep enables ERE syntax by default, users can write patterns like foo|bar or (foo|bar) to match either alternative. It shares the same basic set of command-line options as grep, including flags for case-insensitive matching (-i), inverted matches (-v), line numbering (-n), and more.

Usage examples include: egrep 'error|warning' logfile, which matches lines containing either term; egrep -i 'fail|deny' /var/log/system.log to ignore case; or egrep -v '^#' config.txt to exclude commented lines.

Status within standards: POSIX defines grep with an option to enable extended regular expressions (grep -E), but a separate egrep utility is not required by the standard. As a result, egrep is often considered a compatibility alias, and portable scripts are encouraged to use grep -E for portability across implementations. See also grep and fgrep.