diff --git a/lib/crontab-parse.spec.ts b/lib/crontab-parse.spec.ts
new file mode 100644
index 0000000..0941dda
--- /dev/null
+++ b/lib/crontab-parse.spec.ts
@@ -0,0 +1,191 @@
+import {
+  assertArrayIncludes,
+  assertEquals,
+  assertInstanceOf,
+  assertObjectMatch,
+} from "https://deno.land/std@0.204.0/assert/mod.ts";
+import { astCrontabParse } from "./crontab-parse.ts";
+
+Deno.test("Parse AST", () => {
+  assertEquals(
+    astCrontabParse("* * * * *"),
+    {
+      type: "Statement",
+      start: 0,
+      end: 9,
+      body: [
+        {
+          type: 'AnyValue',
+          start: 0,
+          end: 1,
+        },
+        {
+          end: 3,
+          start: 2,
+          type: "AnyValue",
+        },
+        {
+          end: 5,
+          start: 4,
+          type: "AnyValue",
+        },
+        {
+          end: 7,
+          start: 6,
+          type: "AnyValue",
+        },
+        {
+          end: 9,
+          start: 8,
+          type: "AnyValue",
+        },
+      ]
+    }
+  )
+})
+
+Deno.test("Parse AST", () => {
+  assertEquals(
+    astCrontabParse("1 * * * *").body.at(0),
+    {
+      type: 'NumberValue',
+      start: 0,
+      end: 1,
+      value: 1
+    }
+  );
+  assertObjectMatch(
+    astCrontabParse("1,2,3,4 * * * *"),
+    {
+      body: [{
+        values: [
+          { type: 'NumberValue', value: 1 },
+          { type: 'NumberValue', value: 2 },
+          { type: 'NumberValue', value: 3 },
+          { type: 'NumberValue', value: 4 },
+        ]
+      }]
+    }
+  );
+  assertObjectMatch(
+    astCrontabParse("1,2,3,4 * * * *"),
+    {
+      body: [{
+        values: [
+          { type: 'NumberValue', value: 1 },
+          { type: 'NumberValue', value: 2 },
+          { type: 'NumberValue', value: 3 },
+          { type: 'NumberValue', value: 4 },
+        ]
+      }]
+    }
+  );
+  assertObjectMatch(
+    astCrontabParse("1,2,3,4 * * * *"),
+    {
+      body: [{
+        type: 'ListValue',
+        values: [
+          { type: 'NumberValue', value: 1 },
+          { type: 'NumberValue', value: 2 },
+          { type: 'NumberValue', value: 3 },
+          { type: 'NumberValue', value: 4 },
+        ]
+      }]
+    }
+  );
+  assertObjectMatch(
+    astCrontabParse("1 * * * *"),
+    {
+      body: [{
+        type: 'NumberValue',
+        value: 1
+      }]
+    }
+  );
+  assertObjectMatch(
+    astCrontabParse("1/2 * * * *"),
+    {
+      body: [{
+        type: 'StepValue',
+        left: { type: "NumberValue", value: 1 },
+        right: { type: "NumberValue", value: 2 },
+      }]
+    }
+  );
+  assertObjectMatch(
+    astCrontabParse("1-2 * * * *"),
+    {
+      body: [{
+        type: 'RangeValue',
+        left: {
+          type: 'NumberValue',
+          value: 1
+        },
+        right: {
+          type: 'NumberValue',
+          value: 2
+        }
+      }]
+    }
+  );
+  assertObjectMatch(
+    astCrontabParse("1-2/3 * * * *"),
+    {
+      body: [{
+        type: 'StepValue',
+        left: {
+          type: 'RangeValue',
+          left: {
+            type: 'NumberValue',
+            value: 1
+          },
+          right: {
+            type: 'NumberValue',
+            value: 2
+          }
+        },
+        right: {
+          type: 'NumberValue',
+          value: 3
+        }
+      }]
+    }
+  );
+  assertObjectMatch(
+    astCrontabParse("4,1-2/3 * * * *"),
+    {
+      body: [{
+        type: "ListValue",
+        values: [
+          {
+            type: "NumberValue",
+            value: 4
+          },
+          {
+            type: 'StepValue',
+            left: {
+              type: 'RangeValue',
+              left: {
+                type: 'NumberValue',
+                value: 1
+              },
+              right: {
+                type: 'NumberValue',
+                value: 2
+              }
+            },
+            right: {
+              type: 'NumberValue',
+              value: 3
+            }
+          }
+        ]
+      }]
+    }
+  );
+  assertObjectMatch(
+    astCrontabParse("* * * jan *"),
+    { body: [{}, {}, {}, { value: 1 }] },
+  );
+})
diff --git a/lib/crontab-parse.ts b/lib/crontab-parse.ts
new file mode 100644
index 0000000..f4f3ecd
--- /dev/null
+++ b/lib/crontab-parse.ts
@@ -0,0 +1,332 @@
+type Expression = {
+  start: number
+  end: number
+}
+
+type AnyValueExpression = { type: "AnyValue" } & Expression
+type NumberValueExpression = { type: "NumberValue", value: number } & Expression
+type ListItemExpression = { type: "ListItemValue", value: string } & Expression
+type ListValueExpression = { type: "ListValue", values: MathExpression[] } & Expression
+type StepValueExpression = { type: "StepValue", left: MathExpression, right: MathExpression } & Expression
+type RangeValueExpression = { type: "RangeValue", left: MathExpression, right: MathExpression } & Expression
+
+type MathExpression = AnyValueExpression | NumberValueExpression | ListValueExpression | StepValueExpression | RangeValueExpression
+
+type StatementExpression = {
+  type: "Statement"
+  body: MathExpression[]
+} & Expression
+
+type ExpressionPart = Expression & {
+  body: string
+}
+
+function* eachExpressionPart(body: string): Generator<ExpressionPart> {
+  const exp = /\s+/g
+  let res: null | RegExpExecArray = null
+
+  let nextStart = 0
+  while ((res = exp.exec(body)) !== null) {
+    yield {
+      start: nextStart,
+      end: res.index,
+      body: body.substring(nextStart, res.index)
+    }
+    nextStart = res.index + res[0].length
+  }
+  yield {
+    start: nextStart,
+    end: body.length,
+    body: body.substring(nextStart, body.length)
+  }
+}
+
+type TestExpressionPart = (expressionPart: ExpressionPart) => MathExpression | null
+
+const AnyValueExpressionPartParse: TestExpressionPart = (expressionPart: ExpressionPart): MathExpression | null => expressionPart.body === '*' ? { type: 'AnyValue', start: expressionPart.start, end: expressionPart.end } : null
+
+const toNumber = (value: unknown, enumList?: (string | null)[]): null | number => {
+  if (typeof value !== 'string') return null
+  if (/^\d+$/.test(value)) return Number(value)
+  if (enumList) {
+    let n = -1
+    for (const match of enumList) {
+      n += 1
+      if (match?.toLowerCase() === value.toLowerCase()) {
+        return n
+      }
+    }
+  }
+  return null
+}
+
+const NumberValueExpressionPartParse = (start?: number, end?: number, enumList?: (null | string)[]): TestExpressionPart => (expressionPart: ExpressionPart): MathExpression | null => {
+  const num = toNumber(expressionPart.body, enumList)
+  if (num === null) return null;
+  if (start !== undefined && num < start) return null
+  if (end !== undefined && num > end) return null
+  return { type: 'NumberValue', start: expressionPart.start, end: expressionPart.end, value: num };
+}
+
+const RangeValueExpressionPartParse = (leftTestExpressionPart: TestExpressionPart, rightTestExpressionPart: TestExpressionPart): TestExpressionPart => (expressionPart) => {
+  const expr = /^(.+)\-(.+)$/
+  const regExpExecArray = expr.exec(expressionPart.body)
+  if (!regExpExecArray) return null
+  const [, leftRaw, rightRaw] = regExpExecArray
+  const leftExpressionPart: ExpressionPart = {
+    body: leftRaw,
+    start: expressionPart.start,
+    end: expressionPart.start + leftRaw.length
+  }
+  const rightExpressionPart: ExpressionPart = {
+    body: rightRaw,
+    start: expressionPart.start + leftRaw.length + 1,
+    end: expressionPart.start + leftRaw.length + 1 + rightRaw.length
+  }
+  const left = leftTestExpressionPart(leftExpressionPart)
+  const right = rightTestExpressionPart(rightExpressionPart)
+
+  if (!left) throw new Error('Unexpected token', { cause: leftExpressionPart })
+  if (!right) throw new Error('Unexpected token', { cause: rightExpressionPart })
+
+  return {
+    type: 'RangeValue',
+    start: right!.start,
+    end: left!.end,
+    left,
+    right,
+  }
+}
+
+const StepValueExpressionPartParse = (leftTestExpressionPart: TestExpressionPart, rightTestExpressionPart: TestExpressionPart): TestExpressionPart => (expressionPart) => {
+  const expr = /^(.+)\/(.+)$/
+
+  const regExpExecArray = expr.exec(expressionPart.body)
+
+  if (regExpExecArray === null) return null
+
+  const leftExpressionPart: ExpressionPart = {
+    start: expressionPart.start,
+    end: expressionPart.start + regExpExecArray[1].length,
+    body: regExpExecArray[1],
+  }
+  const rightExpressionPart: ExpressionPart = {
+    start: expressionPart.start + regExpExecArray[1].length + 1,
+    end: expressionPart.start + regExpExecArray[1].length + 1 + regExpExecArray[2].length,
+    body: regExpExecArray[2],
+  }
+
+  const left = leftTestExpressionPart(leftExpressionPart)
+  const right = rightTestExpressionPart(rightExpressionPart)
+  if (!right) throw new Error(`Unexpected token`, { cause: rightExpressionPart })
+  if (!left) throw new Error(`Unexpected token`, { cause: leftExpressionPart })
+
+  return {
+    type: "StepValue",
+    left,
+    right,
+    start: left.start,
+    end: right.end,
+  }
+}
+
+function* splitTokens(body: string): Generator<ListItemExpression> {
+  const spl = /,/g
+
+  let res: null | RegExpExecArray = null
+  let start = 0
+  let end = 0
+  while ((res = spl.exec(body)) !== null) {
+    yield {
+      type: "ListItemValue",
+      start,
+      end,
+      value: body.substring(start, res.index)
+    }
+    start = res.index + 1
+  }
+
+  yield {
+    type: "ListItemValue",
+    start,
+    end: body.length,
+    value: body.substring(start, body.length)
+  }
+
+}
+
+const ListValuesExpressionPartParse = (allowTests: TestExpressionPart[]): TestExpressionPart => (expressionPart) => {
+  if (!/,/.test(expressionPart.body)) return null
+  return {
+    type: 'ListValue',
+    start: expressionPart.start,
+    end: expressionPart.end,
+    values: Array.from(
+      splitTokens(expressionPart.body),
+      item => matrixExpressionPart(allowTests)({
+        start: expressionPart.start + item.start,
+        end: expressionPart.start + item.end,
+        body: item.value
+      })
+    )
+  }
+}
+
+const matrixExpressionPart = (tests: TestExpressionPart[]) => (expressionPart: ExpressionPart): MathExpression => {
+  for (const test of tests) {
+    const res = test(expressionPart)
+    if (res) return res
+  }
+  throw new Error(`Unexpected token`, { cause: expressionPart })
+}
+
+namespace minuteExpressionPartParse {
+  export const number = NumberValueExpressionPartParse(0, 59)
+  export const any = AnyValueExpressionPartParse
+  export const range = RangeValueExpressionPartParse(number, number)
+  export const step = StepValueExpressionPartParse(
+    matrixExpressionPart([
+      number,
+      range,
+    ]),
+    number
+  )
+  export const list = ListValuesExpressionPartParse([
+    step,
+    range,
+    number,
+  ])
+
+  export const all = matrixExpressionPart([
+    list,
+    step,
+    range,
+    any,
+    number,
+  ])
+}
+
+namespace hourExpressionPartParse {
+  export const number = NumberValueExpressionPartParse(0, 23)
+  export const any = AnyValueExpressionPartParse
+  export const range = RangeValueExpressionPartParse(number, number)
+  export const step = StepValueExpressionPartParse(
+    matrixExpressionPart([
+      number,
+      range,
+    ]),
+    number
+  )
+  export const list = ListValuesExpressionPartParse([
+    step,
+    range,
+    number,
+  ])
+
+  export const all = matrixExpressionPart([
+    list,
+    step,
+    range,
+    any,
+    number,
+  ])
+}
+
+namespace dayOfMonthExpressionPartParse {
+  export const number = NumberValueExpressionPartParse(1, 31)
+  export const any = AnyValueExpressionPartParse
+  export const range = RangeValueExpressionPartParse(number, number)
+  export const step = StepValueExpressionPartParse(
+    matrixExpressionPart([
+      number,
+      range,
+    ]),
+    number
+  )
+  export const list = ListValuesExpressionPartParse([
+    step,
+    range,
+    number,
+  ])
+
+  export const all = matrixExpressionPart([
+    list,
+    step,
+    range,
+    any,
+    number,
+  ])
+}
+namespace monthExpressionPartParse {
+  export const number = NumberValueExpressionPartParse(1, 12, [null, 'Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'])
+  export const any = AnyValueExpressionPartParse
+  export const range = RangeValueExpressionPartParse(number, number)
+  export const step = StepValueExpressionPartParse(
+    matrixExpressionPart([
+      number,
+      range,
+    ]),
+    number
+  )
+  export const list = ListValuesExpressionPartParse([
+    step,
+    range,
+    number,
+  ])
+
+  export const all = matrixExpressionPart([
+    list,
+    step,
+    range,
+    any,
+    number,
+  ])
+}
+namespace dayWeekExpressionPartParse {
+  export const number = NumberValueExpressionPartParse(0, 6, ['mon', 'tue', 'wed', 'thu', 'fri', 'sat', 'sun'])
+  export const any = AnyValueExpressionPartParse
+  export const range = RangeValueExpressionPartParse(number, number)
+  export const step = StepValueExpressionPartParse(
+    matrixExpressionPart([
+      number,
+      range,
+    ]),
+    number
+  )
+  export const list = ListValuesExpressionPartParse([
+    step,
+    range,
+    number,
+  ])
+
+  export const all = matrixExpressionPart([
+    list,
+    step,
+    range,
+    any,
+    number,
+  ])
+}
+
+const parseMinuteExpressionPart = (expressionPart: ExpressionPart): MathExpression => minuteExpressionPartParse.all(expressionPart)
+const parseHourExpressionPart = (expressionPart: ExpressionPart): MathExpression => hourExpressionPartParse.all(expressionPart)
+const parseDayMonthExpressionPart = (expressionPart: ExpressionPart): MathExpression => dayOfMonthExpressionPartParse.all(expressionPart)
+const parseMonthExpressionPart = (expressionPart: ExpressionPart): MathExpression => monthExpressionPartParse.all(expressionPart)
+const parseDayWeekExpressionPart = (expressionPart: ExpressionPart): MathExpression => dayWeekExpressionPartParse.all(expressionPart)
+
+const parseExpressionPart = (expressionPart: ExpressionPart, index: number): MathExpression => {
+  const parser = [parseMinuteExpressionPart, parseHourExpressionPart, parseDayMonthExpressionPart, parseMonthExpressionPart, parseDayWeekExpressionPart].at(index)
+  if (!parser) throw new Error(`Unexpected token`, { cause: expressionPart })
+  return parser(expressionPart)
+}
+
+export const astCrontabParse = (body: string): StatementExpression => {
+  const parts = Array.from(eachExpressionPart(body), parseExpressionPart)
+
+  return {
+    type: "Statement",
+    start: 0,
+    end: body.length,
+    body: parts
+  }
+}