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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,
there is no difference in available functionality between the basic and
extended syntaxes.
In other implementations, basic regular expressions are less powerful.
The following description applies to extended regular expressions;
differences for basic regular expressions are summarized afterwards.
Perl-compatible regular expressions give additional functionality, and
are documented in the pcresyntax(3) and pcrepattern(3) manual
pages, but work only if PCRE is available in the system.