It tells grep to use the Extended regular expressions.  Further down in 
the man page for grep it defines the various regular expressions as:

"grep understands three different versions of regular expression syntax:
“basic” (BRE), “extended” (ERE) and “perl” (PRCE). In GNU grep, there is
no difference in available functionality between
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 regular expressions
give additional functionality, and are documented in pcresyntax(3) and
pcrepattern(3), but only work if pcre is available
in the system."

So the current versions of grep treat the basic and extended regular 
expressions as the same, so if they aren't powerful enough you will want 
to use the -P flag to get the PERL regular expressions.

Here's a good web page that will show you the differences between basic 
and extended regular expressions:
http://www.regular-expressions.info/posix.html

Brian Cluff

On 08/24/2014 02:21 PM, Michael Havens wrote:
> what does the -E option do. The man page leaves much to be desired....
> :-)~MIKE~(-:
>
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