diff --git a/index.js b/index.js
index 52fab56..040e26a 100644
--- a/index.js
+++ b/index.js
@@ -3,6 +3,6 @@
 var scan = require('./scan')
 var parse = require('./parse')
 
-module.exports = function (source) {
-  return parse(scan(source))
+module.exports = function (source, options) {
+  return parse(scan(source), options)
 }
diff --git a/parse.js b/parse.js
index a4a52ce..4b36e47 100644
--- a/parse.js
+++ b/parse.js
@@ -1,11 +1,19 @@
 'use strict'
 
+var licenses = []
+  .concat(require('spdx-license-ids'))
+  .concat(require('spdx-license-ids/deprecated'))
+var exceptions = require('spdx-exceptions')
+
 // The ABNF grammar in the spec is totally ambiguous.
 //
 // This parser follows the operator precedence defined in the
 // `Order of Precedence and Parentheses` section.
-
-module.exports = function (tokens) {
+//
+// options:
+//  - Set `relaxed` to `true` to accept invalid license or exception IDs.
+module.exports = function (tokens, options) {
+  options = options || {}
   var index = 0
 
   function hasMore () {
@@ -34,7 +42,10 @@ module.exports = function (tokens) {
   function parseWith () {
     if (parseOperator('WITH')) {
       var t = token()
-      if (t && t.type === 'EXCEPTION') {
+      if (t && t.type === 'IDENTIFIER') {
+        if (!options.relaxed && exceptions.indexOf(t.string) === -1) {
+          throw new Error('`' + t.string + '` is not a valid exception name')
+        }
         next()
         return t.string
       }
@@ -67,8 +78,14 @@ module.exports = function (tokens) {
 
   function parseLicense () {
     var t = token()
-    if (t && t.type === 'LICENSE') {
+    if (t && t.type === 'IDENTIFIER') {
       next()
+      if (licenses.indexOf(t.string) === -1) {
+        if (options.relaxed) {
+          return {noassertion: t.string}
+        }
+        throw new Error('`' + t.string + '` is not a valid license name')
+      }
       var node = {license: t.string}
       if (parseOperator('+')) {
         node.plus = true
diff --git a/scan.js b/scan.js
index d0567f4..a146d19 100644
--- a/scan.js
+++ b/scan.js
@@ -1,10 +1,5 @@
 'use strict'
 
-var licenses = []
-  .concat(require('spdx-license-ids'))
-  .concat(require('spdx-license-ids/deprecated'))
-var exceptions = require('spdx-exceptions')
-
 module.exports = function (source) {
   var index = 0
 
@@ -82,22 +77,11 @@ module.exports = function (source) {
   }
 
   function identifier () {
-    var begin = index
     var string = idstring()
-
-    if (licenses.indexOf(string) !== -1) {
-      return {
-        type: 'LICENSE',
-        string: string
-      }
-    } else if (exceptions.indexOf(string) !== -1) {
-      return {
-        type: 'EXCEPTION',
-        string: string
-      }
+    return string && {
+      type: 'IDENTIFIER',
+      string: string
     }
-
-    index = begin
   }
 
   // Tries to read the next token. Returns `undefined` if no token is
diff --git a/test/index.js b/test/index.js
index 46318fa..6688f97 100644
--- a/test/index.js
+++ b/test/index.js
@@ -83,3 +83,36 @@ it('parses `AND`, `OR` and `WITH` with the correct precedence', function () {
     }
   )
 })
+
+it('rejects invalid license and exception names by default', function () {
+  assert.throws(
+    function () { p('unknownLicense') },
+    /`unknownLicense` is not a valid license name/
+  )
+
+  assert.throws(
+    function () { p('MIT WITH unknownException') },
+    /`unknownException` is not a valid exception name/
+  )
+})
+
+it('accepts invalid license and exception names in relaxed mode', function () {
+  assert.deepEqual(
+    p('unknownLicense', {relaxed: true}),
+    {noassertion: 'unknownLicense'}
+  )
+
+  assert.deepEqual(
+    p('MIT WITH unknownException', {relaxed: true}),
+    {license: 'MIT', exception: 'unknownException'}
+  )
+
+  assert.deepEqual(
+    p('MIT OR Commercial', {relaxed: true}),
+    {
+      left: {license: 'MIT'},
+      conjunction: 'or',
+      right: {noassertion: 'Commercial'}
+    }
+  )
+})