This is not always a good approach. You could write some RegExp code to accept the time string without the colon and another RegExp code to format the string. Remember you will need to validate the range of values for hours to be only 0 - 12 and minutes 0 -59. You may also need to provide for the entry on hours only or minutes only.
How are you going to account for the possibility of entering a value like 111am?
Is this 11:01 am or 1:00 am?