diff --git a/parser.ts b/parser.ts
new file mode 100644
index 0000000..dcbf19d
--- /dev/null
+++ b/parser.ts
@@ -0,0 +1,69 @@
+import { isNotEmpty, isString, isUndefined, trim } from "./deps.ts";
+import type { IntRange, RangeSpec, SuffixRange } from "./types.ts";
+
+const RangeSpecifierRe =
+  /^(?<rangeUnit>([!#$%&'*+-.^_`|~A-Za-z0-9])+)=(?<rangeSet>((([0-9])+-(([0-9])+)?)|(-([0-9])+))((\x20|\t)*,(\x20|\t)*((([0-9])+-(([0-9])+)?)|(-([0-9])+)))*)$/;
+
+export interface RangesSpecifier {
+  readonly rangeUnit: string;
+  readonly rangeSet: string;
+}
+
+export function parseRangesSpecifier(input: string): RangesSpecifier {
+  const result = RangeSpecifierRe.exec(input);
+
+  if (!result || !result.groups) throw Error();
+
+  const rangeUnit = result.groups.rangeUnit;
+  const rangeSet = result.groups.rangeSet;
+
+  if (isUndefined(rangeUnit) || isUndefined(rangeSet)) {
+    throw SyntaxError();
+  }
+
+  return { rangeUnit, rangeSet };
+}
+
+const RangeSpecRe =
+  /^((?<firstPos>[0-9]+)-(?<lastPos>[0-9]+)?)$|^(-(?<suffixLength>[0-9]+))$/;
+
+export function parseRangeSpec(input: string): IntRange | SuffixRange {
+  const result = RangeSpecRe.exec(input);
+
+  if (!result || !result.groups) {
+    throw SyntaxError("syntax error");
+  }
+
+  const firstPos = result.groups.firstPos;
+  const lastPos = result.groups.lastPos;
+  const suffixLength = result.groups.suffixLength;
+
+  if (isString(firstPos)) {
+    return {
+      firstPos: Number.parseInt(firstPos),
+      lastPos: lastPos ? Number.parseInt(lastPos) : undefined,
+    };
+  }
+
+  if (!isString(suffixLength)) {
+    throw SyntaxError("conflict syntax");
+  }
+
+  const suffix = Number.parseInt(suffixLength);
+
+  if (isNaN(suffix)) throw SyntaxError();
+
+  return { suffixLength: suffix };
+}
+
+export function parseRangeSet(input: string): [RangeSpec, ...RangeSpec[]] {
+  const result = input.split(",");
+
+  const ranges = result
+    .map(trim)
+    .map(parseRangeSpec);
+
+  if (!isNotEmpty(ranges)) throw SyntaxError();
+
+  return ranges;
+}
diff --git a/parser_test.ts b/parser_test.ts
new file mode 100644
index 0000000..d512fd7
--- /dev/null
+++ b/parser_test.ts
@@ -0,0 +1,94 @@
+import {
+  parseRangeSpec,
+  parseRangesSpecifier,
+  RangesSpecifier,
+} from "./parser.ts";
+import { RangeSpec } from "./types.ts";
+import { assertEquals, assertThrows, describe, it } from "./_dev_deps.ts";
+
+describe("parseRangesSpecifier", () => {
+  it("should return parsed <ranges-specifier>", () => {
+    const table: [string, RangesSpecifier][] = [
+      ["bytes=0-100", { rangeUnit: "bytes", rangeSet: "0-100" }],
+      ["bytes=0-", { rangeUnit: "bytes", rangeSet: "0-" }],
+      ["bytes=-100", { rangeUnit: "bytes", rangeSet: "-100" }],
+      ["bytes=0-0,1-1", { rangeUnit: "bytes", rangeSet: "0-0,1-1" }],
+      ["bytes=-100,0-100", { rangeUnit: "bytes", rangeSet: "-100,0-100" }],
+      ["bytes=-100 , 0-100", { rangeUnit: "bytes", rangeSet: "-100 , 0-100" }],
+      ["bytes=-100 , -200   , 300-400", {
+        rangeUnit: "bytes",
+        rangeSet: "-100 , -200   , 300-400",
+      }],
+      ["unknown!=-1234567890", {
+        rangeUnit: "unknown!",
+        rangeSet: "-1234567890",
+      }],
+    ];
+
+    table.forEach(([input, expected]) => {
+      assertEquals(parseRangesSpecifier(input), expected);
+    });
+  });
+
+  it("should throw error if the input is invalid syntax", () => {
+    const table: string[] = [
+      "",
+      "a",
+      "abc",
+      "a=b",
+      "=",
+      "a=1",
+      "<>=1-",
+      "a=1.1",
+      "a=1.0",
+      "a=120 ",
+      " a=120",
+      " a=120 ",
+      "a1=120",
+    ];
+
+    table.forEach((input) => {
+      assertThrows(() => parseRangesSpecifier(input));
+    });
+  });
+});
+
+describe("parseRangeSpec", () => {
+  it("should return parsed <range-spec>", () => {
+    const table: [string, RangeSpec][] = [
+      ["0-", { firstPos: 0, lastPos: undefined }],
+      ["0-0", { firstPos: 0, lastPos: 0 }],
+      ["100-100", { firstPos: 100, lastPos: 100 }],
+      ["100-0", { firstPos: 100, lastPos: 0 }],
+      ["100-0", { firstPos: 100, lastPos: 0 }],
+
+      ["-0", { suffixLength: 0 }],
+      ["-100", { suffixLength: 100 }],
+    ];
+
+    table.forEach(([input, expected]) => {
+      assertEquals(parseRangeSpec(input), expected);
+    });
+  });
+
+  it("should throw error if the input is invalid syntax", () => {
+    const table: string[] = [
+      "",
+      "a",
+      "0",
+      "1",
+      "1.0-",
+      "0.1-",
+      "0-0.0",
+      "0-0.1",
+      "100-100,",
+      "100- ",
+      " 100-",
+      "-100,",
+    ];
+
+    table.forEach((input) => {
+      assertThrows(() => parseRangeSpec(input));
+    });
+  });
+});