REGULAR EXPRESSIONS(from `man grep`)

       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” (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 may not be available on every system.