diff --git a/BigInteger.d.ts b/BigInteger.d.ts
index 4709e0a..76b8409 100644
--- a/BigInteger.d.ts
+++ b/BigInteger.d.ts
@@ -23,8 +23,11 @@ declare namespace bigInt {
 
         /**
          * Parse a string into a bigInt.
+         * Default base is 10.
+         * Default alphabet is "0123456789abcdefghijklmnopqrstuvwxyz".
+         * caseSensitive defaults to false.
          */
-        (string: string, base?: BigNumber): BigInteger;
+        (string: string, base?: BigNumber, alphabet?: string, caseSensitive?: boolean): BigInteger;
 
         /**
          * no-op.
@@ -66,12 +69,12 @@ declare namespace bigInt {
         /**
          * Equivalent to bigInt(-1).
          */
-        minusOne:  BigInteger;
+        minusOne: BigInteger;
 
         /**
          * Equivalent to bigInt(1).
          */
-        one:  BigInteger;
+        one: BigInteger;
 
         /**
          * Returns a random number between min and max.
@@ -130,7 +133,7 @@ declare namespace bigInt {
          * Performs division and returns an object with two properties: quotient and remainder.
          * The sign of the remainder will match the sign of the dividend.
          */
-        divmod(number: BigNumber): {quotient: BigInteger, remainder: BigInteger};
+        divmod(number: BigNumber): { quotient: BigInteger, remainder: BigInteger };
 
         /**
          * Alias for the equals method.
@@ -346,7 +349,7 @@ declare namespace bigInt {
          *
          * Converts a bigInt to an object representing it as an array of integers module the given radix.
          */
-         toArray(radix: number): BaseArray;
+        toArray(radix: number): BaseArray;
 
         /**
          * Converts a bigInt into a native Javascript number. Loses precision for numbers outside the range.
@@ -357,7 +360,7 @@ declare namespace bigInt {
          * Converts a bigInt to a string.
          */
         toString(radix?: number): string;
-		
+
 		/**
          * Converts a bigInt to a string. This method is called behind the scenes in JSON.stringify.
          */
diff --git a/BigInteger.js b/BigInteger.js
index 46a7bcc..1fa9c23 100644
--- a/BigInteger.js
+++ b/BigInteger.js
@@ -5,13 +5,13 @@ var bigInt = (function (undefined) {
         LOG_BASE = 7,
         MAX_INT = 9007199254740992,
         MAX_INT_ARR = smallToArray(MAX_INT),
-        LOG_MAX_INT = Math.log(MAX_INT);
+        DEFAULT_ALPHABET = "0123456789abcdefghijklmnopqrstuvwxyz";
 
     var supportsNativeBigInt = typeof BigInt === "function";
 
-    function Integer(v, radix) {
+    function Integer(v, radix, alphabet, caseSensitive) {
         if (typeof v === "undefined") return Integer[0];
-        if (typeof radix !== "undefined") return +radix === 10 ? parseValue(v) : parseBase(v, radix);
+        if (typeof radix !== "undefined") return +radix === 10 && !alphabet ? parseValue(v) : parseBase(v, radix, alphabet, caseSensitive);
         return parseValue(v);
     }
 
@@ -1164,42 +1164,40 @@ var bigInt = (function (undefined) {
         }
         return low.add(Integer.fromArray(result, BASE, false));
     }
-    var parseBase = function (text, base) {
+
+    var parseBase = function (text, base, alphabet, caseSensitive) {
+        alphabet = alphabet || DEFAULT_ALPHABET;
+        text = String(text);
+        if (!caseSensitive) {
+            text = text.toLowerCase();
+            alphabet = alphabet.toLowerCase();
+        }
         var length = text.length;
         var i;
         var absBase = Math.abs(base);
-        for (var i = 0; i < length; i++) {
-            var c = text[i].toLowerCase();
+        var alphabetValues = {};
+        for (i = 0; i < alphabet.length; i++) {
+            alphabetValues[alphabet[i]] = i;
+        }
+        for (i = 0; i < length; i++) {
+            var c = text[i];
             if (c === "-") continue;
-            if (/[a-z0-9]/.test(c)) {
-                if (/[0-9]/.test(c) && +c >= absBase) {
+            if (c in alphabetValues) {
+                if (alphabetValues[c] >= absBase) {
                     if (c === "1" && absBase === 1) continue;
                     throw new Error(c + " is not a valid digit in base " + base + ".");
-                } else if (c.charCodeAt(0) - 87 >= absBase) {
-                    throw new Error(c + " is not a valid digit in base " + base + ".");
-                }
-            }
-        }
-        if (2 <= base && base <= 36) {
-            if (length <= LOG_MAX_INT / Math.log(base)) {
-                var result = parseInt(text, base);
-                if (isNaN(result)) {
-                    throw new Error(c + " is not a valid digit in base " + base + ".");
                 }
-                return parseValue(parseInt(text, base));
             }
         }
         base = parseValue(base);
         var digits = [];
         var isNegative = text[0] === "-";
         for (i = isNegative ? 1 : 0; i < text.length; i++) {
-            var c = text[i].toLowerCase(),
-                charCode = c.charCodeAt(0);
-            if (48 <= charCode && charCode <= 57) digits.push(parseValue(c));
-            else if (97 <= charCode && charCode <= 122) digits.push(parseValue(c.charCodeAt(0) - 87));
+            var c = text[i];
+            if (c in alphabetValues) digits.push(parseValue(alphabetValues[c]));
             else if (c === "<") {
                 var start = i;
-                do { i++; } while (text[i] !== ">");
+                do { i++; } while (text[i] !== ">" && i < text.length);
                 digits.push(parseValue(text.slice(start + 1, i)));
             }
             else throw new Error(c + " is not a valid character");
@@ -1216,9 +1214,10 @@ var bigInt = (function (undefined) {
         return isNegative ? val.negate() : val;
     }
 
-    function stringify(digit) {
-        if (digit <= 35) {
-            return "0123456789abcdefghijklmnopqrstuvwxyz".charAt(digit);
+    function stringify(digit, alphabet) {
+        alphabet = alphabet || DEFAULT_ALPHABET;
+        if (digit < alphabet.length) {
+            return alphabet[digit];
         }
         return "<" + digit + ">";
     }
@@ -1278,9 +1277,11 @@ var bigInt = (function (undefined) {
         return { value: out.reverse(), isNegative: neg };
     }
 
-    function toBaseString(n, base) {
+    function toBaseString(n, base, alphabet) {
         var arr = toBase(n, base);
-        return (arr.isNegative ? "-" : "") + arr.value.map(stringify).join('');
+        return (arr.isNegative ? "-" : "") + arr.value.map(function (x) {
+            return stringify(x, alphabet);
+        }).join('');
     }
 
     BigInteger.prototype.toArray = function (radix) {
@@ -1295,9 +1296,9 @@ var bigInt = (function (undefined) {
         return toBase(this, radix);
     };
 
-    BigInteger.prototype.toString = function (radix) {
+    BigInteger.prototype.toString = function (radix, alphabet) {
         if (radix === undefined) radix = 10;
-        if (radix !== 10) return toBaseString(this, radix);
+        if (radix !== 10) return toBaseString(this, radix, alphabet);
         var v = this.value, l = v.length, str = String(v[--l]), zeros = "0000000", digit;
         while (--l >= 0) {
             digit = String(v[l]);
@@ -1307,9 +1308,9 @@ var bigInt = (function (undefined) {
         return sign + str;
     };
 
-    SmallInteger.prototype.toString = function (radix) {
+    SmallInteger.prototype.toString = function (radix, alphabet) {
         if (radix === undefined) radix = 10;
-        if (radix != 10) return toBaseString(this, radix);
+        if (radix != 10) return toBaseString(this, radix, alphabet);
         return String(this.value);
     };
 
diff --git a/README.md b/README.md
index fe1507a..0901973 100644
--- a/README.md
+++ b/README.md
@@ -27,7 +27,7 @@ Then you can include it in your code:
 
 
 ## Usage
-### `bigInt(number, [base])`
+### `bigInt(number, [base], [alphabet], [caseSensitive])`
 
 You can create a bigInt by calling the `bigInt` function. You can pass in
 
@@ -36,7 +36,11 @@ You can create a bigInt by calling the `bigInt` function. You can pass in
  - another bigInt.
  - nothing, and it will return `bigInt.zero`.
 
- If you provide a second parameter, then it will parse `number` as a number in base `base`. Note that `base` can be any bigInt (even negative or zero). The letters "a-z" and "A-Z" will be interpreted as the numbers 10 to 35. Higher digits can be specified in angle brackets (`<` and `>`).
+ If you provide a second parameter, then it will parse `number` as a number in base `base`. Note that `base` can be any bigInt (even negative or zero). The letters "a-z" and "A-Z" will be interpreted as the numbers 10 to 35. Higher digits can be specified in angle brackets (`<` and `>`). The default `base` is `10`.
+
+ You can specify a custom alphabet for base conversion with the third parameter. The default `alphabet` is `"0123456789abcdefghijklmnopqrstuvwxyz"`.
+
+ The fourth parameter specifies whether or not the number string should be case-sensitive, i.e. whether `a` and `A` should be treated as different digits. By default `caseSensitive` is `false`.
 
 Examples:
 
@@ -520,13 +524,17 @@ Returns a random number between `min` and `max`.
 
 ### Override Methods
 
-#### `toString(radix = 10)`
+#### `toString(radix = 10, [alphabet])`
 
 Converts a bigInt to a string. There is an optional radix parameter (which defaults to 10) that converts the number to the given radix. Digits in the range `10-35` will use the letters `a-z`.
 
  - `bigInt("1e9").toString()` => `"1000000000"`
  - `bigInt("1e9").toString(16)` => `"3b9aca00"`
 
+ You can use a custom base alphabet with the second parameter. The default `alphabet` is `"0123456789abcdefghijklmnopqrstuvwxyz"`.
+
+  - `bigInt("5").toString(2, "aA")` => `"AaA"`
+
 **Note that arithmetical operators will trigger the `valueOf` function rather than the `toString` function.** When converting a bigInteger to a string, you should use the `toString` method or the `String` function instead of adding the empty string.
 
  - `bigInt("999999999999999999").toString()` => `"999999999999999999"`
diff --git a/spec/spec.js b/spec/spec.js
index d0071dd..cea9885 100644
--- a/spec/spec.js
+++ b/spec/spec.js
@@ -1066,6 +1066,13 @@ describe("BigInteger", function () {
 
             expect(function () { return bigInt(1).toArray(0); }).toThrow();
         });
+
+        it("allows custom alphabet", function () {
+            expect(bigInt("9786534201", 10, "9786534201")).toEqualBigInt("0123456789");
+            expect(bigInt("bC", 3, "abc")).toEqualBigInt("5");
+            expect(bigInt("AAa", 2, "aA", true)).toEqualBigInt("6");
+            expect(bigInt("10").toString(2, "Aa")).toEqual("aAaA");
+        });
     });
 
     describe("Bitwise operations", function () {