Parsing a BigRational efficiently

I'm using the BigRational beta off the BCL CodePlex (bcl.codeplex.com), and I realized it had no parsing method, so I tried to write one. However, it's quite inefficient (5.5ms for a 254 character number). That's over 45x slower than the BigInteger implementation. I'd like to lower it to .5ms if that's even possible.

private static Regex DigitDotDigit = new Regex(@"^(\-|\+)?(\d+\.\d+)$", RegexOptions.Compiled); private static Regex PlainDigit = new Regex(@"^(\-|\+)?\d+$", RegexOptions.Compiled); private static Regex DigitSlashDigit = new Regex(@"^(\-|\+)?\d+/\d+$", RegexOptions.Compiled); private static Regex DotDigit = new Regex(@"^(\-|\+)?(\.\d+)", RegexOptions.Compiled); private static bool RegexInitiated = false; public static bool TryParse(string parse, out BigRational result) { if (DigitDotDigit.IsMatch(parse)) { int zeros; bool isNegative = false; string[] parts = parse.TrimStart('+').Split('.'); parts[1] = parts[1].TrimEnd('0'); if (parts[0].StartsWith("-")) { isNegative = true; parts[0] = parts[0].Substring(1); } BigRational whole = new BigRational(BigInteger.Parse(parts[0]), BigInteger.Zero, BigInteger.One); BigRational decimalPart = new BigRational(BigInteger.Parse(parts[1]), BigInteger.Zero, BigInteger.One); zeros = parts[1].Length - parts[1].TrimStart('0').Length; toSubtract = toSubtract + zeros; if (zeros > 0) { toSubtract = toSubtract - 1; } while (toSubtract != 0) { decimalPart /= 10; toSubtract = toSubtract - 1; } result = whole + decimalPart; if (isNegative) { result = -result; } return true; } else if (DotDigit.IsMatch(parse)) { return TryParse("0" + parse, out result); } else if (PlainDigit.IsMatch(parse)) { parse = parse.TrimStart('+'); if (parse.StartsWith("-")) { result = new BigRational(-BigInteger.Parse(parse), BigInteger.Zero, BigInteger.One); return true; } result = new BigRational(BigInteger.Parse(parse), BigInteger.Zero, BigInteger.One); return true; } else if (DigitSlashDigit.IsMatch(parse)) { string[] parts = parse.TrimStart('+').Split('/'); if (parts[0].StartsWith("-")) { parts[0] = parts[0].Substring(1); result = -(new BigRational(BigInteger.Parse(parts[0]), BigInteger.Parse(parts[1]))); return true; } result = new BigRational(BigInteger.Parse(parts[0]), BigInteger.Parse(parts[1])); return true; } result = BigInteger.Zero; return false; }