spdx-expression-parse

This package parses SPDX license expression strings describing license terms, like package.json license strings, into consistently structured ECMAScript objects. The npm command-line interface depends on this package, as do many automatic license-audit tools.
In a nutshell:

var parse = require('spdx-expression-parse')
var assert = require('assert')

assert.deepEqual(
  // Licensed under the terms of the Two-Clause BSD License.
  parse('BSD-2-Clause'),
  {license: 'BSD-2-Clause'}
)

assert.throws(function () {
  // An invalid SPDX license expression.
  // Should be `Apache-2.0`.
  parse('Apache 2')
})

assert.deepEqual(
  // Dual licensed under either:
  // - LGPL 2.1
  // - a combination of Three-Clause BSD and MIT
  parse('(LGPL-2.1 OR BSD-3-Clause AND MIT)'),
  {
    left: {license: 'LGPL-2.1'},
    conjunction: 'or',
    right: {
      left: {license: 'BSD-3-Clause'},
      conjunction: 'and',
      right: {license: 'MIT'}
    }
  }
)

The syntax comes from the Software Package Data eXchange (SPDX), a standard from the Linux Foundation for shareable data about software package license terms. SPDX aims to make sharing and auditing license data easy, especially for users of open-source software.
The bulk of the SPDX standard describes syntax and semantics of XML metadata files. This package implements two lightweight, plain-text components of that larger standard:

1. The license list, a mapping from specific string identifiers, like Apache-2.0, to standard form license texts and bolt-on license exceptions. The spdx-license-ids and spdx-exceptions packages implement the license list. spdx-expression-parse depends on and require()s them.

Any license identifier from the license list is a valid license expression:

```javascript
var identifiers = []
  .concat(require('spdx-license-ids'))
  .concat(require('spdx-license-ids/deprecated'))
  .filter(function (id) { return id[id.length - 1] !== '+' })

identifiers.forEach(function (id) {
  assert.deepEqual(parse(id), {license: id})
})
```

So is any license identifier `WITH` a standardized license exception:

```javascript
identifiers.forEach(function (id) {
  require('spdx-exceptions').forEach(function (e) {
    assert.deepEqual(
      parse(id + ' WITH ' + e),
      {license: id, exception: e}
    )
  })
})
```

1. The license expression language, for describing simple and complex license terms, like MIT for MIT-licensed and (GPL-2.0 OR Apache-2.0) for dual-licensing under GPL 2.0 and Apache 2.0. spdx-expression-parse itself implements license expression language, exporting a parser.

```javascript
assert.deepEqual(
  // Licensed under a combination of:
  // - the MIT License AND
  // - a combination of:
  //   - LGPL 2.1 (or a later version) AND
  //   - Three-Clause BSD
  parse('(MIT AND (LGPL-2.1+ AND BSD-3-Clause))'),
  {
    left: {license: 'MIT'},
    conjunction: 'and',
    right: {
      left: {license: 'LGPL-2.1', plus: true},
      conjunction: 'and',
      right: {license: 'BSD-3-Clause'}
    }
  }
)
```

This package differs slightly from the SPDX standard in allowing lower- and mixed-case AND, OR, and WITH operators:

assert.deepEqual(
  parse('MIT or BSD-2-Clause'),
  { left: { license: 'MIT' }, conjunction: 'or', right: { license: 'BSD-2-Clause' } }
)
assert.deepEqual(
  parse('GPL-2.0 with GCC-exception-2.0'),
  { license: 'GPL-2.0', exception: 'GCC-exception-2.0' }
)

The Linux Foundation and its contributors license the SPDX standard under the terms of the Creative Commons Attribution License 3.0 Unported (SPDX: "CC-BY-3.0"). "SPDX" is a United States federally registered trademark of the Linux Foundation. The authors of this package license their work under the terms of the MIT License.