validation for Time HH:MM am or pm Format using regular expression

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I am working on web application using asp.net with vb code.I have a textbox for time field for which i am using regular expression validator.the format i want is HH:MM am. the regular expression i am using is "(0[1-9]|[1][0-2])[:]" + "(0[0-9]|[1-5][0-9])[ ][A|a|P|p][M|m]"i am entering the time example: 08:30 AM or 08:30 PM but the regular expression is showing error message.Can anyone help me with correct regular expression.thankyou all in advanceshubha