@c Copyright (C) 2009-2011 Sergey Poznyakoff
@c Permission is granted to copy, distribute and/or modify this document
@c under the terms of the GNU Free Documentation License, Version 1.3 or
@c any later version published by the Free Software Foundation; with no
@c Invariant Sections, with the Front-Cover texts being ``Mailfromd Manual'',
@c and with the Back-Cover Texts at your option. 
@cindex s-expression
An @dfn{s-expression} is a @command{sed}-like transformation expression:

@smallexample
s/@var{regexp}/@var{replace}/[@var{flags}]
@end smallexample

@noindent
where @var{regexp} is a @dfn{regular expression}, @var{replace} is a
replacement for each part of the input that matches @var{regexp}.  Both
@var{regexp} and @var{replace} are described in detail in
@ref{The "s" Command, The "s" Command, The `s' Command, sed, GNU sed}.

As in @command{sed}, you can give several replace expressions,
separated by a semicolon.

Supported @var{flags} are:

@table @samp
@cindex g, @option{transform} flag
@item g
Apply the replacement to @emph{all} matches to the @var{regexp}, not
just the first.

@cindex i, @option{transform} flag
@item i
Use case-insensitive matching

@cindex x, @option{transform} flag
@item x
@var{regexp} is an @dfn{extended regular expression} (@pxref{Extended
regexps, Extended regular expressions, Extended regular expressions,
sed, GNU sed}).

@item @var{number}
Only replace the @var{number}th match of the @var{regexp}.

Note: the @acronym{POSIX} standard does not specify what should happen
when you mix the @samp{g} and @var{number} modifiers.  @command{Mailfromd}
follows the GNU @command{sed} implementation in this regard, so
the interaction is defined to be: ignore matches before the
@var{number}th, and then match and replace all matches from the
@var{number}th on.
@end table

Any delimiter can be used in lieue of @samp{/}, the only requirement being
that it be used consistently throughout the expression. For example,
the following two expressions are equivalent:

@smallexample
@group
s/one/two/
s,one,two,
@end group
@end smallexample

  Changing delimiters is often useful when the @var{regex} contains
slashes.  For instance, it is more convenient to write @code{s,/,-,} than
@code{s/\//-/}.