Using Vbscript Validation Tab to format/change a text type property.

Here is a script that runs a similar check. It should be easy for you to change it to suit your needs.

'Script will check that Deadline is entered as a valid date in the format DD.MM in a text property
Option Explicit
If not IsNull( PropertyValue.TypedValue ) Then
	Dim szPropVal : szPropVal = PropertyValue.GetValueAsUnlocalizedText
	If len(szPropVal) > 5 then
		err.raise MFScriptCancel, "Deadline must be entered as 5 characters in the format DD.MM (day.month)"
	Elseif not IsNumeric(left(szPropVal,2)) Then
		err.raise MFScriptCancel, "Deadline must be entered as 5 characters in the format DD.MM (day.month)" & VbCrLF & "Day must be 01 - 31"
	Elseif left(szPropVal,1) > 3 Then
		err.raise MFScriptCancel, "Deadline must be entered as 5 characters in the format DD.MM (day.month)" & VbCrLF & "First character must be 0, 1, 2 or 3"
	Elseif left(szPropVal,2) > 31 Then
		err.raise MFScriptCancel, "Deadline must be entered as 5 characters in the format DD.MM (day.month)" & VbCrLF & "Day must be 01 - 31"
	Elseif mid(szPropVal,3,1) <> "." Then
		err.raise MFScriptCancel, "Deadline must be entered as 5 characters in the format DD.MM (day.month)" & VbCrLF & "Third character must be '.'"
	Elseif not IsNumeric(mid(szPropVal,4,2)) Then
		err.raise MFScriptCancel, "Deadline must be entered as 5 characters in the format DD.MM (day.month)" & VbCrLF & "Month must be 01 - 12"
	Elseif mid(szPropVal,4,1) > 1 Then
		err.raise MFScriptCancel, "Deadline must be entered as 5 characters in the format DD.MM (day.month)" & VbCrLF & "Fourth character must be 0 or 1"
	Elseif mid(szPropVal,4,2) > 12 Then
		err.raise MFScriptCancel, "Deadline must be entered as 5 characters in the format DD.MM (day.month)" & VbCrLF & "Month must be 01 - 12"
	End if
End if