Mailfromd
	General-Purpose Mail Filter	Sergey Poznyakoff

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C. S-Expression

An s-expression is a sed-like transformation expression:

s/regexp/replace/[flags]

where regexp is a regular expression, replace is a replacement for each part of the input that matches regexp. Both regexp and replace are described in detail in The "s" Command: (sed)The "s" Command section `The `s' Command' in GNU sed.

As in sed, you can give several replace expressions, separated by a semicolon.

Supported flags are:

‘g’

Apply the replacement to all matches to the regexp, not just the first.

‘i’

Use case-insensitive matching

‘x’

regexp is an extended regular expression (see Extended regular expressions: (sed)Extended regexps section `Extended regular expressions' in GNU sed).

‘number’

Only replace the numberth match of the regexp.

Note: the POSIX standard does not specify what should happen when you mix the ‘g’ and number modifiers. Mailfromd follows the GNU sed implementation in this regard, so the interaction is defined to be: ignore matches before the numberth, and then match and replace all matches from the numberth on.

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

s/one/two/
s,one,two,

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