Often a single license can be used to represent the licensing terms of a source code or binary file, but there are situations where a single license identifier is not sufficient. A common example is when software is offered under a choice of one or more licenses (e.g., GPL-2.0-only OR BSD-3-Clause). Another example is when a set of licenses is needed to represent a binary program constructed by compiling and linking two (or more) different source files each governed by different licenses (e.g., LGPL-2.1-only AND BSD-3-Clause).
SPDX License Expressions provide a way for one to construct expressions that more accurately represent the licensing terms typically found in open source software source code. A license expression could be a single license identifier found on the SPDX License List; a user defined license reference denoted by the LicenseRef-[idString]; a license identifier combined with an SPDX exception; or some combination of license identifiers, license references and exceptions constructed using a small set of defined operators (e.g., AND, OR, WITH and +). We provide the definition of what constitutes a valid an SPDX License Expression in this section.
The exact syntax of license expressions is described below in ABNF.
idstring = 1*(ALPHA / DIGIT / "-" / "." )
license-id = <short form license identifier in Annex A.1>
license-exception-id = <short form license exception identifier in Annex A.2>
license-ref = ["DocumentRef-"1\*(idstring)":"]"LicenseRef-"1*(idstring)
simple-expression = license-id / license-id"+" / license-ref
compound-expression = 1*1(simple-expression /
simple-expression "WITH" license-exception-id /
compound-expression "AND" compound-expression /
compound-expression "OR" compound-expression /
"(" compound-expression ")" )
license-expression = 1*1(simple-expression / compound-expression)
In the following sections we describe in more detail <license-expression> construct, a licensing expression string that enables a more accurate representation of the licensing terms of modern-day software.
A valid <license-expression> string consists of either:
(i) a simple license expression, such as a single license identifier; or
(ii) a more complex expression constructed by combining smaller valid expressions using Boolean license operators.
There MUST NOT be white space between a license-id and any following +. This supports easy parsing and backwards compatibility. There MUST be white space on either side of the operator "WITH". There MUST be white space and/or parentheses on either side of the operators AND and OR.
In the tag:value format, a license expression MUST be on a single line, and MUST NOT include a line break in the middle of the expression.
License expression operators (AND, OR and WITH) should be matched in a case-sensitive manner.
License identifiers (including license exception identifiers) used in SPDX documents or source code files should be matched in a case-insensitive manner. In other words, MIT, Mit and mIt should all be treated as the same identifier and referring to the same license.
However, please be aware that it is often important to match with the case of the canonical identifier on the SPDX License List. This is because the canonical identifier's case is used in the URL of the license's or exception's entry on the List, and because the canonical identifier is translated to a URI in RDF documents.
A simple <license-expression> is composed one of the following:
- An SPDX License List Short Form Identifier. For example: CDDL-1.0
- An SPDX License List Short Form Identifier with a unary "+" operator suffix to represent the current version of the license or any later version. For example: CDDL-1.0+
- An SPDX user defined license reference: ["DocumentRef-"1*(idstring)":"]"LicenseRef-"1*(idstring)
Some examples:
LicenseRef-23
LicenseRef-MIT-Style-1
DocumentRef-spdx-tool-1.2:LicenseRef-MIT-Style-2
More expressive composite license expressions can be constructed using "OR", "AND", and "WITH" operators similar to constructing mathematical expressions using arithmetic operators.
For the tag:value format, any license expression that consists of more than one license identifier and/or LicenseRef, may optionally be encapsulated by parentheses: "( )".
Nested parentheses can also be used to specify an order of precedence which is discussed in more detail in D.4.5.
If presented with a choice between two or more licenses, use the disjunctive binary "OR" operator to construct a new license expression, where both the left and right operands are valid license expression values.
For example, when given a choice between the LGPL-2.1-only or MIT licenses, a valid expression would be:
LGPL-2.1-only OR MIT
An example representing a choice between three different licenses would be:
LGPL-2.1-only OR MIT OR BSD-3-Clause
If required to simultaneously comply with two or more licenses, use the conjunctive binary "AND" operator to construct a new license expression, where both the left and right operands are a valid license expression values.
For example, when one is required to comply with both the LGPL-2.1-only or MIT licenses, a valid expression would be:
LGPL-2.1-only AND MIT
An example where all three different licenses apply would be:
LGPL-2.1-only AND MIT AND BSD-2-Clause
Sometimes a set of license terms apply except under special circumstances. In this case, use the binary "WITH" operator to construct a new license expression to represent the special exception situation. A valid <license-expression> is where the left operand is a <simple-expression> value and the right operand is a <license-exception-id> that represents the special exception terms.
For example, when the Bison exception is to be applied to GPL-2.0-or-later, the expression would be:
GPL-2.0-or-later WITH Bison-exception-2.2
The current set of valid exceptions can be found in Annex A.2. For the most up to date set of exceptions please see spdx.org/licenses. If the applicable exception is not found on the SPDX License Exception List, then use a single <license-ref> to represent the entire license terms (including the exception).
The order of application of the operators in an expression matters (similar to mathematical operators). The default operator order of precedence of a <license-expression> a is:
+
WITH
AND
OR
where a lower order operator is applied before a higher order operator.
For example, the following expression:
LGPL-2.1-only OR BSD-3-Clause AND MIT
represents a license choice between either LGPL-2.1-only and the expression BSD-3-Clause AND MIT because the AND operator takes precedence over (is applied before) the OR operator.
When required to express an order of precedence that is different from the default order a <license-expression> can be encapsulated in pairs of parentheses: ( ), to indicate that the operators found inside the parentheses takes precedence over operators outside. This is also similar to the use of parentheses in an algebraic expression e.g., (5+7)/2.
For instance, the following expression:
MIT AND (LGPL-2.1-or-later OR BSD-3-Clause)
states the OR operator should be applied before the AND operator. That is, one should first select between the LGPL-2.1-or-later or the BSD-3-Clause license before applying the MIT license.
A conjunctive license can be expressed in RDF via a <spdx:ConjunctiveLicenseSet> element, with an spdx:member property for each element in the conjunctive license. Two or more members are required.
<spdx:ConjunctiveLicenseSet>
<spdx:member rdf:resource="http://spdx.org/licenses/GPL-2.0-only"/>
<spdx:ExtractedLicensingInfo rdf:about
="http://example.org#LicenseRef-EternalSurrender">
<spdx:extractedText>
In exchange for using this software, you agree to give
its author all your worldly possessions. You will not
hold the author liable for all the damage this software
will inevitably cause not only to your person and
property, but to the entire fabric of the cosmos.
</spdx:extractedText>
<spdx:licenseId>LicenseRef-EternalSurrender</spdx:licenseId>
</spdx:ExtractedLicensingInfo>
</spdx:ConjunctiveLicenseSet>
A disjunctive license can be expressed in RDF via a <spdx:DisjunctiveLicenseSet> element, with an spdx:member property for each element in the disjunctive license. Two or more members are required.
<spdx:DisjunctiveLicenseSet>
<spdx:member rdf:resource="http://spdx.org/licenses/GPL-2.0-only"/>
<spdx:member>
<spdx:ExtractedLicensingInfo rdf:about
="http://example.org#LicenseRef-EternalSurrender">
<spdx:extractedText>
In exchange for using this software, you agree to
give its author all your worldly possessions. You
will not hold the author liable for all the damage
this software will inevitably cause not only to
your person and property, but to the entire fabric
of the cosmos.
</spdx:extractedText>
<spdx:licenseId>LicenseRef-EternalSurrender</spdx:licenseId>
</spdx:ExtractedLicensingInfo>
</spdx:member>
</spdx:DisjunctiveLicenseSet>
A License Exception can be expressed in RDF via a <spdx:LicenseException> element. This element has the following unique mandatory (unless specified otherwise) attributes:
comment- Anrdfs:commentelement describing the nature of the exception.seeAlso(optional, one or more)- Anrdfs:seeAlsoelement referencing external sources of information on the exception.example(optional) - Text describing examples of this exception.name- The full human readable name of the item.licenseExceptionId- The identifier of an exception in the SPDX License List to which the exception applies.licenseExceptionText- Full text of the license exception.
<rdf:Description rdf:about
="http://example.org#SPDXRef-ButIdDontWantToException">
<rdfs:comment>This exception may be invalid in some
jurisdictions.</rdfs:comment>
<rdfs:seeAlso>http://dilbert.com/strip/1997-01-15</rdfs:seeAlso>
<spdx:example>So this one time, I had a license exception
…</spdx:example>
<spdx:licenseExceptionText>
A user of this software may decline to follow any subset of
the terms of this license upon finding any or all such terms
unfavorable.
</spdx:licenseExceptionText>
<spdx:name>"But I Don't Want To" Exception</spdx:name>
<spdx:licenseExceptionId>SPDXRef-ButIdDontWantToException</spdx:licenseExceptionId>
<rdf:type rdf:resource
="http://spdx.org/rdf/terms#LicenseException"/>
</rdf:Description>