(grep.info)Regular Expressions

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3 Regular Expressions
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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.

Fundamental Structure

Character Classes and Bracket Expressions

The Backslash Character and Special Expressions

Anchoring

Back-references and Subexpressions

Basic vs Extended