This is a part of the output from "man regexp" on my computer.

Not extremely clear the first time you read it, but should be enough to
start with regular expressions.

I put a couple of examples at the end of this file.

REGULAR EXPRESSION SYNTAX
     A regular expression is zero or more branches, separated by |'.  It
     matches anything that matches one of the branches.

     A branch is zero or more pieces, concatenated.  It matches a match for
     the first, followed by a match for the second, etc.

     A piece is an atom possibly followed by `*', `+', or `?'.  An atom fol-
     lowed by `*' matches a sequence of 0 or more matches of the atom.  An
     atom followed by `+' matches a sequence of 1 or more matches of the atom.
     An atom followed by `?' matches a match of the atom, or the null string.

     An atom is a regular expression in parentheses (matching a match for the
     regular expression), a range (see below), `.'  (matching any single char-
     acter), `^' (matching the null string at the beginning of the input
     string), `$' (matching the null string at the end of the input string), a
     `\' followed by a single character (matching that character), or a single
     character with no other significance (matching that character).

     A range is a sequence of characters enclosed in `[]'.  It normally match-
     es any single character from the sequence.  If the sequence begins with
     `^', it matches any single character not from the rest of the sequence.
     If two characters in the sequence are separated by `-', this is shorthand
     for the full list of ASCII characters between them (e.g. `[0-9]' matches
     any decimal digit).  To include a literal `[' in the sequence, make it
     the first character (following a possible `^').  To include a literal
     `-', make it the first or last character.

A couple of examples
~~~~~~~~~~~~~~~~~~~~
Pattern: dre
Matches: andrew.com

Pattern: jp.*g
Matches: hey.jpg ho.jpyg boo.jpxxxxg

Pattern: jpe+g
Matches: hey.jpeg hey.jpeeg
Dont ma: hey.jpg

Pattern: \.(gif|jpe*g)
Matches: hello.giffe hello.gif banner.jpg banner.jpeg banner.jpeeg

Pattern: \.(gif|jpe*g)$
Matches: hello.gif banner.jpg banner.jpeg
Dont ma: hello.giffe