private void textBox1_Leave(object sender, EventArgs e)
{
  // Will receive the properly formatted dollars and cents string.

  string dollarsAndCents = "";

  // Validate the text string input.  If the string is in a valid
  // format, then convert it to dollars and cents format.  Finally,
  // replace the original string in the textbox with the newly
  // formatted dollars and cents string.

  if (this.ToCurrency(textBox1.Text, ref dollarsAndCents))
    textBox1.Text = dollarsAndCents;
}

private bool ToCurrency(string text, ref string dollarsAndCents)
{
  // Perform some preliminary formatting on the original text string.
  // Specifically, delete any occurrances of the dollar sign ($),
  // commas (,) and blank spaces ( ).

  text = text.Replace("$", "").Replace(",", "").Replace(" ", "");

  // Formulate our regular expression pattern.  It includes one
  // overriding group called currency. Within the currency group,
  // we can have three distinct patterns:
  //
  //   Dollars.Cents
  //   Dollars.
  //          .Cents
  //
  // The decimal point is mandatory in the "Dollars.Cents", and the
  // ".Cents" variants, but is optional in the "Dollars." variant.
  //
  // Once the pattern is formulated, attempt a regular expression
  // match using the text supplied in the textbox.

  string pattern = @"^\s*(?<Currency>(?i:(?<Dollars>\d+)\.(?<Cents>\d{1,2}))|(?<Dollars>\d+)\.*|\.(?<Cents>\d{1,2}))";
  Match m = Regex.Match(text, pattern);

  // If our pattern match was unsuccessful, then the string is not
  // in a valid format.

  if (!m.Success)
    return false;

  // Now that we've validated the input text, let's format it
  // into a proper currency format.  First, use a string builder
  // object to concatenate the dollars portion with a decimal
  // point and finally with the cents portion.

  StringBuilder sb = new StringBuilder();

  if (m.Groups["Dollars"].Success)
    sb.Append(m.Groups["Dollars"].Value);

  sb.Append(".");

  if (m.Groups["Cents"].Success)
    sb.Append(m.Groups["Cents"].Value);

  // Now that we have the dollars, decimal and cents concatenated,
  // let's convert the concatenated string into value of type
  // "double", and then use the .NET framework's own formatting
  // tools to convert it into currency format.

  dollarsAndCents = string.Format("{0:C}", Convert.ToDouble(sb.ToString()));

  // Success.
  return true;
}

string pattern = @"^\s*(?<Currency>(?i:(?<Dollars>\d+)\.(?<Cents>\d{1,2}))|(?<Dollars>\d+)\.*|\.(?<Cents>\d{1,2}))";

string pattern = @"^\s*(?<Currency>(?i:(?<Dollars>\d+)\.(?<Cents>\d+))|(?<Dollars>\d+)\.*|\.(?<Cents>\d+))";