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.
three different versions of regular expression syntax:
basic (BRE), extended (ERE), and Perl-compatible (PCRE).
there is no difference in available functionality between the basic and
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.