Regular Expression to disallow two consecutive white spaces in the middle of a string

I need a regex to meet the following requirements:

• Only letters, periods and whitespace are allowed.
• No white space at the beginning and at the end of the string.
• White space in the middle of the string is OK, but not two consecutive white spaces.

Matches:

"Hello world."
"Hello World. This is ok."

Not Matches:

" Hello World. "
"Hello world 123." 
"Hello  world."

This worked in my case

<asp:RegularExpressionValidator ID="revDescription" runat="server" 
                                ControlToValidate="taDescription" Display="Dynamic" ErrorMessage="Invalid Description." 
                                Text="&nbsp" 
                                ValidationExpression="^(?i)(?![ ])(?!.*[ ]{2})(?!.*[ ]$)[A-Z. ]{8,20}$"></asp:RegularExpressionValidator>

Here's a solution in Python, using anchors and negative lookahead assertions to make sure the whitespace rules are followed:

regex = re.compile(
    """^          # Start of string
    (?![ ])       # Assert no space at the start
    (?!.*[ ]{2})  # Assert no two spaces in the middle
    (?!.*[ ]$)    # Assert no space at the end
    [A-Z. ]{8,20} # Match 8-20 ASCII letters, dots or spaces
    $             # End of string""", 
    re.IGNORECASE | re.VERBOSE)