2 files changed, 73 insertions, 0 deletions
diff --git a/__tests__/lexer.test.js b/__tests__/lexer.test.js
new file mode 100644
index 0000000..20d461f
--- /dev/null
+++ b/__tests__/lexer.test.js
@@ -0,0 +1,29 @@
+const { lex } = require('../src/lexer.js')
+
+describe('lex', () => {
+  it('lexes the empty string', () => {
+    expect(lex('')).toEqual([])
+  })
+
+  it('signals an error on unexpected input', () => {
+    expect(lex('q')).toBe('error')
+  })
+
+  describe('basic dice', () => {
+    it('1d6', () => {
+      expect(lex('1d6')).toEqual([
+        { type: 'number', value: 1 },
+        { type: 'd' },
+        { type: 'number', value: 6 }
+      ])
+    })
+
+    it('42d172', () => {
+      expect(lex('42d172')).toEqual([
+        { type: 'number', value: 42 },
+        { type: 'd' },
+        { type: 'number', value: 172 }
+      ])
+    })
+  })
+})
diff --git a/src/lexer.js b/src/lexer.js
new file mode 100644
index 0000000..ff7b0d4
--- /dev/null
+++ b/src/lexer.js
@@ -0,0 +1,44 @@
+const lexemeTypes = []
+
+const newLexemeType = (type, regex, adder) => {
+  lexemeTypes.push({
+    type,
+    regex: new RegExp(`^(${regex})(.*)$`),
+    adder: adder || (lexemes => { lexemes.push({ type }) })
+  })
+}
+
+const newValueLexeme = (type, regex, converter = v => v) => {
+  newLexemeType(type, regex, (lexemes, value) => {
+    lexemes.push({ type, value: converter(value) })
+  })
+}
+
+newValueLexeme('number', '\\d+', Number)
+newLexemeType('d', 'd')
+
+const lex = (expressionString) => {
+  let lexemes = []
+
+  while (expressionString.length > 0) {
+    let matched = false
+
+    lexemeTypes.forEach(lexemeType => {
+      let matches = lexemeType.regex.exec(expressionString)
+
+      if (matches) {
+        matched = true
+        lexemeType.adder(lexemes, matches[1])
+        expressionString = matches[2]
+      }
+    })
+
+    if (!matched) {
+      return 'error'
+    }
+  }
+
+  return lexemes
+}
+
+exports.lex = lex