File: grep.info,  Node: Regular Expressions,  Next: Usage,  Prev: Invoking,  Up: Top

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’, 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.

* Menu:

* Fundamental Structure::
* Character Classes and Bracket Expressions::
* Special Backslash Expressions::
* Anchoring::
* Back-references and Subexpressions::
* Basic vs Extended::
* Problematic Expressions::
* Character Encoding::
* Matching Non-ASCII::