4 files changed, 49 insertions, 0 deletions
diff --git a/__tests__/lexer.test.js b/__tests__/lexer.test.js
index 6434e17..e9115a1 100644
--- a/__tests__/lexer.test.js
+++ b/__tests__/lexer.test.js
@@ -64,4 +64,28 @@ describe('lex', () => {
       ])
     })
   })
+
+  describe('lexes subtraction', () => {
+    it('1d6 - 1d4', () => {
+      expect(lex('1d6 - 1d4')).toEqual([
+        { type: 'constant', value: 1 },
+        { type: 'd' },
+        { type: 'constant', value: 6 },
+        { type: '-' },
+        { type: 'constant', value: 1 },
+        { type: 'd' },
+        { type: 'constant', value: 4 }
+      ])
+    })
+
+    it('2d17 - 4', () => {
+      expect(lex('2d17 - 4')).toEqual([
+        { type: 'constant', value: 2 },
+        { type: 'd' },
+        { type: 'constant', value: 17 },
+        { type: '-' },
+        { type: 'constant', value: 4 }
+      ])
+    })
+  })
 })
diff --git a/__tests__/parser.test.js b/__tests__/parser.test.js
index b0dc28d..6fe296c 100644
--- a/__tests__/parser.test.js
+++ b/__tests__/parser.test.js
@@ -56,4 +56,20 @@ describe('parse', () => {
       }
     })
   })
+
+  it('parses dice subtraction', () => {
+    expect(parse('1d6 - 2d8')).toEqual({
+      type: 'subtract',
+      left: {
+        type: 'd',
+        left: { type: 'constant', value: 1 },
+        right: { type: 'constant', value: 6 }
+      },
+      right: {
+        type: 'd',
+        left: { type: 'constant', value: 2 },
+        right: { type: 'constant', value: 8 }
+      }
+    })
+  })
 })
diff --git a/src/lexer.js b/src/lexer.js
index 97130d8..fd347c6 100644
--- a/src/lexer.js
+++ b/src/lexer.js
@@ -21,6 +21,7 @@ const newSkippableLexeme = (type, regex) => {
 newValueLexeme('constant', '\\d+', Number)
 newLexemeType('d', 'd')
 newLexemeType('+', '\\+')
+newLexemeType('-', '-')
 newSkippableLexeme('whitespace', '\\s+')
 
 const lex = (expressionString) => {
diff --git a/src/parser.js b/src/parser.js
index c7d91c3..6c39c77 100644
--- a/src/parser.js
+++ b/src/parser.js
@@ -32,6 +32,14 @@ newSymbol('+', null, 20, (left, parser) => {
   }
 })
 
+newSymbol('-', null, 20, (left, parser) => {
+  return {
+    type: 'subtract',
+    left: left,
+    right: parser.expression(20)
+  }
+})
+
 newSymbol('end', null, -1)
 
 const newParser = (tokens) => {