Regular Expressions
===================

For some of the configuration options, a rudimentary knowledge of Perl-
style regular expressions will help you run Majordomo through its
tricks.  A regular expression is a concise way of expressing a pattern
in a series of characters.  The full power of regular expressions can
make some difficult tasks quite easy, but we will only brush the
surface here.

The character / is used to mark the beginning and end of a regular
expression.  Letters and numbers stand for themselves.  Many of the
other characters are symbolic.  Some commonly used ones are:

  \@    the `@' found in nearly all addresses; it must be preceeded by a
        backslash to avoid errors in Perl
  .     (period) any character
  *     previous character, zero or more times; note especially...
  .*    any character, zero or more times
  +     previous character, one or more times; so for example...
  a+    letter "a", one or more times
  \     next character stands for itself; so for example...
  \.     literally a period, not meaning "any character"
  ^     beginning of the string; so for example...
  ^a    a string beginning with letter "a"
  $     end of the string; so for example...
  a$    a string ending with letter "a"


Example 1
---------

     /foo\.example\.com/

Notice that the periods are preceded by a backslash so that they are
interpreted as periods, rather than wildcards.  This matches any string
containing:

     foo.example.com

such as:

     foo.example.com
     bar.foo.example.com
     user@bar.foo.example.com
     users%bar.foo.example.com@example.com


Example 2
---------

     /johndoe\@.*foo\.example\.com/

The `@' has special meaning to Perl and must be prefixed with a backslash
to avoid errors.  The string ".*" means "any character, zero or more
times".  So this matches:

     johndoe@foo.example.com
     johndoe@terminus.foo.example.com
     ajohndoe@terminus.foo.example.com@example.com

But it doesn't match:

     johndoe@example.com
     brent@foo.example.com


Example 3
---------

     /^johndoe\@.*cs\.example\.org$/

This is similar to Example 2, and matches the same first two strings:

     johndoe@foo.example.org
     johndoe@terminus.foo.example.org

But it doesn't match:

     ajohndoe@terminus.foo.example.org@example.com

...because the regular expression says the string has to begin with
letter "j" and end with letter "g", by using the ^ and $ symbols, and
neither of those is true for ajohndoe@terminus.foo.example.org@example.com.


Example 4
---------

     /.*/

This is the regular expression that matches anything.


Example 5
---------

     /.\*johndoe/

Here the * is preceded by a \, so it refers literally to an asterisk
character and not the symbolic meaning "zero or more times".  The . still
has its symbolic meaning of "any one character", so it would match:

     a*johndoe
     s*johndoe

Because the . by itself implies one character, it would not match:

     *johndoe


Example 6
---------

Normally all matches are case sensitive; you can make any match case
insensitive by appending an `i' to the end of the expression.

     /example\.com/i

This would match example.com, EXAMPLE.com, ExAmPlE.cOm, etc.  Removing the `i':

     /example\.com/

...would match example.com but not EXAMPLE.com or any other capitalization.


To be on the safe side put a \ in front of any characters in the
regular expressions that are not numbers or letters.  In order to put
a / into the regular expression, the same rule holds: precede it
with a \.  Thus, with \ in front of the / and = characters, this:

     /\/CO\=US/

...matches /CO=US and may be a useful regular expression to those of you
who need to deal with X.400 addresses that contain / characters.