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Network Working Group K. Homme
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Request for Comments: 5229 University of Oslo
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Updates: 5228 January 2008
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Category: Standards Track
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Sieve Email Filtering: Variables Extension
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This document specifies an Internet standards track protocol for the
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Internet community, and requests discussion and suggestions for
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improvements. Please refer to the current edition of the "Internet
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Official Protocol Standards" (STD 1) for the standardization state
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and status of this protocol. Distribution of this memo is unlimited.
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In advanced mail filtering rule sets, it is useful to keep state or
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configuration details across rules. This document updates the Sieve
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filtering language (RFC 5228) with an extension to support variables.
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The extension changes the interpretation of strings, adds an action
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to store data in variables, and supplies a new test so that the value
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of a string can be examined.
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RFC 5229 Sieve: Variables Extension January 2008
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This is an extension to the Sieve language defined by [SIEVE]. It
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adds support for storing and referencing named data. The mechanisms
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detailed in this document will only apply to Sieve scripts that
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include a require clause for the "variables" extension. The require
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clauses themselves are not affected by this extension.
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Conventions for notations are as in [SIEVE] section 1.1, including
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use of [KEYWORDS] and [ABNF]. The grammar builds on the grammar of
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[SIEVE]. In this document, "character" means a character from the
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ISO 10646 coded character set [ISO10646], which may consist of
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multiple octets coded in [UTF-8], and "variable" is a named reference
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to data stored or read back using the mechanisms of this extension.
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2. Capability Identifier
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The capability string associated with the extension defined in this
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document is "variables".
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3. Interpretation of Strings
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This extension changes the semantics of quoted-string, multi-line-
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literal and multi-line-dotstuff found in [SIEVE] to enable the
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inclusion of the value of variables.
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When a string is evaluated, substrings matching variable-ref SHALL be
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replaced by the value of variable-name. Only one pass through the
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string SHALL be done. Variable names are case insensitive, so "foo"
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and "FOO" refer to the same variable. Unknown variables are replaced
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variable-ref = "${" [namespace] variable-name "}"
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namespace = identifier "." *sub-namespace
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sub-namespace = variable-name "."
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variable-name = num-variable / identifier
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num-variable = 1*DIGIT
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"&%${}!" => unchanged, as the empty string is an illegal
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"${doh!}" => unchanged, as "!" is illegal in identifiers
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The variable "company" holds the value "ACME". No other variables
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"${full}" => the empty string
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"${company}" => "ACME"
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"${BAD${Company}" => "${BADACME"
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"${President, ${Company} Inc.}"
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=> "${President, ACME Inc.}"
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The expanded string MUST use the variable values that are current
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when control reaches the statement the string is part of.
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Strings where no variable substitutions take place are referred to as
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constant strings. Future extensions may specify that passing non-
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constant strings as arguments to its actions or tests is an error.
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Namespaces are meant for future extensions that make internal state
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available through variables. These variables SHOULD be put in a
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namespace whose first component is the same as its capability string.
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Such extensions SHOULD state which, if any, of the variables in its
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namespace are modifiable with the "set" action.
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References to namespaces without a prior require statement for the
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relevant extension MUST cause an error.
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Tests or actions in future extensions may need to access the
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unexpanded version of the string argument and, e.g., do the expansion
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after setting variables in its namespace. The design of the
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implementation should allow this.
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3.1. Quoting and Encoded Characters
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The semantics of quoting using backslash are not changed: backslash
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quoting is resolved before doing variable substitution. Similarly,
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encoded character processing (see Section 2.4.2.4 of [SIEVE]) is
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performed before doing variable substitution, but after quoting.
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"${fo\o}" => ${foo} => the expansion of variable foo.
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"${fo\\o}" => ${fo\o} => illegal identifier => left verbatim.
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"\${foo}" => ${foo} => the expansion of variable foo.
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"\\${foo}" => \${foo} => a backslash character followed by the
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expansion of variable foo.
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If it is required to include a character sequence such as
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"${beep}" verbatim in a text literal, the user can define a
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variable to circumvent expansion to the empty string.
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set "text" "regarding ${dollar}{beep}";
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require ["encoded-character", "variables"];
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set "name" "Ethelbert"
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if header :contains "Subject" "dear${hex:20 24 7b 4e}ame}" {
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# the test string is "dear Ethelbert"
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A "match variable" has a name consisting only of decimal digits and
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has no namespace component.
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The decimal value of the match variable name will index the list of
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matching strings from the most recently evaluated successful match of
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type ":matches". The list is empty if no match has been successful.
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Note: Extra leading zeroes are allowed and ignored.
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The list will contain one string for each wildcard ("?" and "*") in
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the match pattern. Each string holds the substring from the source
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value that the corresponding wildcard expands to, possibly the empty
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string. The wildcards match as little as possible (non-greedy
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The first string in the list has index 1. If the index is out of
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range, the empty string will be substituted. Index 0 contains the
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matched part of the source value.
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The interpreter MUST short-circuit tests, i.e., not perform more
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tests than necessary to find the result. Evaluation order MUST be
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left to right. If a test has two or more list arguments, the
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implementation is free to choose which to iterate over first.
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An extension describing a new match type (e.g., [REGEX]) MAY specify
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that match variables are set as a side effect when the match type is
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used in a script that has enabled the "variables" extension.
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require ["fileinto", "variables"];
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if header :matches "List-ID" "*<*@*" {
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fileinto "INBOX.lists.${2}"; stop;
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# Subject: [acme-users] [fwd] version 1.0 is out
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if header :matches "Subject" "[*] *" {
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# ${1} will hold "acme-users",
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# ${2} will hold "[fwd] version 1.0 is out"
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fileinfo "INBOX.lists.${1}"; stop;
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# To: coyote@ACME.Example.COM
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if address :matches ["To", "Cc"] ["coyote@**.com",
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# ${0} is the matching address
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# ${1} is always the empty string
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# ${2} is part of the domain name ("ACME.Example")
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fileinto "INBOX.business.${2}"; stop;
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# Control wouldn't reach this block if any match was
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# successful, so no match variables are set at this
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if anyof (true, address :domain :matches "To" "*.com") {
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# The second test is never evaluated, so there are
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# still no match variables set.
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Usage: set [MODIFIER] <name: string> <value: string>
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The "set" action stores the specified value in the variable
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identified by name. The name MUST be a constant string and conform
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to the syntax of variable-name. Match variables cannot be set. A
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namespace cannot be used unless an extension explicitly allows its
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use in "set". An invalid name MUST be detected as a syntax error.
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Modifiers are applied on a value before it is stored in the variable.
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See the next section for details.
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Variables are only visible to the currently running script. Note:
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Future extensions may provide different scoping rules for variables.
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Variable names are case insensitive.
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set "honorific" "Mr";
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set "first_name" "Wile";
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set "last_name" "Coyote";
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Dear ${HONORIFIC} ${last_name},
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I'm out, please leave a message after the meep.
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"set" does not affect the implicit keep. It is compatible with all
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actions defined in [SIEVE].
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Usage: ":lower" / ":upper" / ":lowerfirst" / ":upperfirst" /
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":quotewildcard" / ":length"
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Modifier names are case insensitive. Unknown modifiers MUST yield a
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syntax error. More than one modifier can be specified, in which case
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they are applied according to this precedence list, largest value
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+--------------------------------+
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| Precedence Modifier |
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+--------------------------------+
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+--------------------------------+
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+--------------------------------+
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| 20 :quotewildcard |
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+--------------------------------+
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+--------------------------------+
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It is an error to use two or more modifiers of the same precedence in
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a single "set" action.
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# The value assigned to the variable is printed after the arrow
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set "a" "juMBlEd lETteRS"; => "juMBlEd lETteRS"
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set :length "b" "${a}"; => "15"
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set :lower "b" "${a}"; => "jumbled letters"
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set :upperfirst "b" "${a}"; => "JuMBlEd lETteRS"
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set :upperfirst :lower "b" "${a}"; => "Jumbled letters"
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set :quotewildcard "b" "Rock*"; => "Rock\*"
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4.1.1. Modifier ":length"
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The value is the decimal number of characters in the expansion,
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converted to a string.
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4.1.2. Modifier ":quotewildcard"
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This modifier adds the necessary quoting to ensure that the expanded
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text will only match a literal occurrence if used as a parameter to
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:matches. Every character with special meaning ("*", "?", and "\")
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is prefixed with "\" in the expansion.
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4.1.3. Case Modifiers
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These modifiers change the letters of the text from upper to lower
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case or vice versa. Characters other than "A"-"Z" and "a"-"z" from
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US-ASCII are left unchanged.
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4.1.3.1. Modifier ":upper"
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All lower case letters are converted to their upper case
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4.1.3.2. Modifier ":lower"
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All upper case letters are converted to their lower case
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4.1.3.3. Modifier ":upperfirst"
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The first character of the string is converted to upper case if it is
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a letter and set in lower case. The rest of the string is left
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4.1.3.4. Modifier ":lowerfirst"
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The first character of the string is converted to lower case if it is
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a letter and set in upper case. The rest of the string is left
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Usage: string [MATCH-TYPE] [COMPARATOR]
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<source: string-list> <key-list: string-list>
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The "string" test evaluates to true if any of the source strings
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matches any key. The type of match defaults to ":is".
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In the "string" test, both source and key-list are taken from the
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script, not the message, and whitespace stripping MUST NOT be done
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unless the script explicitly requests this through some future
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set "state" "${state} pending";
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if string :matches " ${state} " "* pending *" {
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# the above test always succeeds
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The "relational" extension [RELATIONAL] adds a match type called
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":count". The count of a single string is 0 if it is the empty
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string, or 1 otherwise. The count of a string list is the sum of the
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counts of the member strings.
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6. Implementation Limits
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An implementation of this document MUST support at least 128 distinct
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variables. The supported length of variable names MUST be at least
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32 characters. Each variable MUST be able to hold at least 4000
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characters. Attempts to set the variable to a value larger than what
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the implementation supports SHOULD be reported as an error at
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compile-time if possible. If the attempt is discovered during run-
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time, the value SHOULD be truncated, and it MUST NOT be treated as an
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Match variables ${1} through ${9} MUST be supported. References to
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higher indices than those the implementation supports MUST be treated
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as a syntax error, which SHOULD be discovered at compile-time.
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7. Security Considerations
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When match variables are used, and the author of the script isn't
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careful, strings can contain arbitrary values controlled by the
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Since values stored by "set" that exceed implementation limits are
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silently truncated, it's not appropriate to store large structures
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with security implications in variables.
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The introduction of variables makes advanced decision making easier
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to write, but since no looping construct is provided, all Sieve
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scripts will terminate in an orderly manner.
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Sieve filtering should not be relied on as a security measure against
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hostile mail messages. Sieve is designed to do simple, mostly static
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tests, and is not suitable for use as a spam or virus checker, where
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the perpetrator has a motivation to vary the format of the mail in
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order to avoid filtering rules. See also [SPAMTEST].
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8. IANA Considerations
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The following template specifies the IANA registration of the
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variables Sieve extension specified in this document:
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Subject: Registration of new Sieve extension
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Capability name: variables
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Description: Adds support for variables to the Sieve filtering
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Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>
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Thanks to Cyrus Daboo, Jutta Degener, Ned Freed, Lawrence Greenfield,
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Jeffrey Hutzelman, Mark E. Mallett, Alexey Melnikov, Peder Stray, and
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Nigel Swinson for valuable feedback.
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10.1. Normative References
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[ABNF] Crocker, D., Ed., and Overell, P., "Augmented BNF for
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Syntax Specifications: ABNF", RFC 4234, October 2005.
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[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
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Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RELATIONAL] Segmuller, W. and B. Leiba, "Sieve Email Filtering:
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Relational Extension", RFC 5231, January 2008.
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[SIEVE] Guenther, P., Ed., and T. Showalter, Ed., "Sieve: An
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Email Filtering Language", RFC 5228, January 2008.
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[UTF-8] Yergeau, F., "UTF-8, a transformation format of Unicode
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and ISO 10646", RFC 3629, November 2003.
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10.2. Informative References
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[ISO10646] ISO/IEC, "Information Technology - Universal Multiple-
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Octet Coded Character Set (UCS) - Part 1: Architecture
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and Basic Multilingual Plane", May 1993, with
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[REGEX] Murchison, K., "Sieve Email Filtering -- Regular
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Expression Extension", Work in Progress, February 2006.
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[SPAMTEST] Daboo, C., "Sieve Email Filtering: Spamtest and
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Virustest Extensions", RFC 5235, January 2008.
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EMail: kjetilho@ifi.uio.no
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Full Copyright Statement
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Copyright (C) The IETF Trust (2008).
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This document is subject to the rights, licenses and restrictions
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contained in BCP 78, and except as set forth therein, the authors
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retain all their rights.
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This document and the information contained herein are provided on an
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"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
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OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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Intellectual Property
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The IETF takes no position regarding the validity or scope of any
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Intellectual Property Rights or other rights that might be claimed to
587
pertain to the implementation or use of the technology described in
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this document or the extent to which any license under such rights
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might or might not be available; nor does it represent that it has
590
made any independent effort to identify any such rights. Information
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on the procedures with respect to rights in RFC documents can be
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found in BCP 78 and BCP 79.
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Copies of IPR disclosures made to the IETF Secretariat and any
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attempt made to obtain a general license or permission for the use of
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such proprietary rights by implementers or users of this
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specification can be obtained from the IETF on-line IPR repository at
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http://www.ietf.org/ipr.
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The IETF invites any interested party to bring to its attention any
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rights that may cover technology that may be required to implement
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this standard. Please address the information to the IETF at
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