A regular expression is a pattern that describes a set of strings.
Regular expressions are constructed analogously to arithmetic expressions,
by using various operators to combine smaller expressions.
grep understands
three different versions of regular expression syntax:
basic (BRE), extended (ERE), and Perl-compatible (PCRE).
In GNU grep,
basic and extended regular expressions are merely different notations
for the same pattern-matching functionality.
In other implementations, basic regular expressions are ordinarily
less powerful than extended, though occasionally it is the other way around.
The following description applies to extended regular expressions;
differences for basic regular expressions are summarized afterwards.
Perl-compatible regular expressions have different functionality, and
are documented in the pcre2syntax(3) and pcre2pattern(3) manual
pages, but work only if PCRE is available in the system.