From 3f01cccb53b67814aee7080d221cbabfe56d94af Mon Sep 17 00:00:00 2001
From: Nathan <nathan@users.noreply.github.com>
Date: Sat, 18 Aug 2018 10:23:41 -0400
Subject: [PATCH 1/3] Fix reUnion() for empty regex lists

Return (?!), which never matches anything.
---
 moo.js | 1 +
 1 file changed, 1 insertion(+)

diff --git a/moo.js b/moo.js
index 2cdb0d2..d1939e6 100644
--- a/moo.js
+++ b/moo.js
@@ -28,6 +28,7 @@
     return '(' + s + ')'
   }
   function reUnion(regexps) {
+    if (!regexps.length) return '(?!)'
     var source =  regexps.map(function(s) {
       return "(?:" + s + ")"
     }).join('|')

From 6ee26215b8289c3624a0528836c3b685cbfe9436 Mon Sep 17 00:00:00 2001
From: Nathan <nathan@users.noreply.github.com>
Date: Sat, 18 Aug 2018 10:34:01 -0400
Subject: [PATCH 2/3] Test: handles empty rule set

---
 test/test.js | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/test/test.js b/test/test.js
index 3d811fc..58c866e 100644
--- a/test/test.js
+++ b/test/test.js
@@ -10,7 +10,15 @@ function lexAll(lexer) {return Array.from(lexer)}
 
 describe('compiler', () => {
 
-  // TODO handles empty rule set
+  test('handles empty rule set', () => {
+    const lex = compile({})
+    lex.reset('nope!')
+    expect(() => lex.next()).toThrow('invalid syntax')
+
+    const lex2 = compile({err: moo.error})
+    lex2.reset('nope!')
+    expect(lex2.next()).toMatchObject({type: 'err', text: 'nope!'})
+  })
 
   test("warns for /g, /y, /i, /m", () => {
     expect(() => compile({ word: /foo/ })).not.toThrow()

From bc9ff58aa0330ac0c481a6520dcc369f2e195dcb Mon Sep 17 00:00:00 2001
From: Nathan <nathan@users.noreply.github.com>
Date: Sat, 18 Aug 2018 10:35:26 -0400
Subject: [PATCH 3/3] Test: stateful compiler handles empty rule set

---
 test/test.js | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/test/test.js b/test/test.js
index 58c866e..461b0f8 100644
--- a/test/test.js
+++ b/test/test.js
@@ -18,6 +18,14 @@ describe('compiler', () => {
     const lex2 = compile({err: moo.error})
     lex2.reset('nope!')
     expect(lex2.next()).toMatchObject({type: 'err', text: 'nope!'})
+
+    const lex3 = moo.states({main: {}})
+    lex3.reset('nope!')
+    expect(() => lex3.next()).toThrow('invalid syntax')
+
+    const lex4 = moo.states({main: {err: moo.error}})
+    lex4.reset('nope!')
+    expect(lex4.next()).toMatchObject({type: 'err', text: 'nope!'})
   })
 
   test("warns for /g, /y, /i, /m", () => {