Regular Expressions in .NET

using System;
using System.Text.RegularExpressions;

namespace TimeParserRegEx
{
	/// <summary>
	/// Summary description for Class1.
	/// </summary>
	class FreeFormTimeParser
	{
		/// <summary>
		/// The main entry point for the application.
		/// </summary>
		[STAThread]
		static void Main(string[] args)
		{
			try 
			{
				System.Console.WriteLine ("Enter time to be parsed: ");
				string inputStr = System.Console.ReadLine ();
				DateTime parsedTime = FreeFormTimeParser.ParseTime (inputStr);

				System.Console.WriteLine ("The parsed time in long notation is: " 
					+ parsedTime.ToLongTimeString());
			} 
			catch (FormatException e) 
			{
				System.Console.WriteLine ("Invalid time format");
			}
		}

		private const string TIME_STR = 
			@"^\s?(?<time>"
			+ @"(?<hour>\d{1,2})"
			+ @"(:(?<min>\d{1,2}))?"
			+ @"\s?((?<am_pm>(am|pm)))?"
			+ @")\s?$";

		private static int ConvertAmPm (string amPm, int hour)
		{
			int retHour = hour;

			amPm = amPm.ToLower();
			if (amPm.Equals("am")) 
			{
				// all hours remain the same except the 12:00 am (which is 0000 hours)
				if (hour == 12)
					retHour = 00;
			}
			else if (amPm.Equals("pm")) 
			{
				// add 12 to hours except if 12:00 pm
				if (hour != 12)
					retHour = hour + 12;
			}
			else
				throw new FormatException ("Invalid amPm flag format");

			return retHour;
		}

		static DateTime ParseTime (string strTime)
		{
			DateTime currTime = DateTime.Now;
			DateTime finalTime = DateTime.Today;
			Match m;
			int hour = 0,
				min = 0;

			Regex regExTime = new Regex (TIME_STR, 
				RegexOptions.IgnoreCase
				| RegexOptions.CultureInvariant
				| RegexOptions.IgnorePatternWhitespace
				| RegexOptions.Compiled);

			m = regExTime.Match (strTime);
			if (m.Success)
			{
				if (m.Groups["hour"].Success)
					hour = Int32.Parse (m.Groups["hour"].Value);
				if (m.Groups["min"].Success)
					min = Int32.Parse (m.Groups["min"].Value);
				if (m.Groups["am_pm"].Success) 
				{
					hour = ConvertAmPm (m.Groups["am_pm"].Value, hour);
				}
			} 
			else
				throw new FormatException ("Invalid time format");

			if (hour > 23 || min > 59)
				throw new FormatException ("Invalid time format");

			finalTime = new DateTime (currTime.Year, currTime.Month, currTime.Day,
				hour, min, 0);

			return finalTime;
		}
	}
}