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

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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>

Answer 1

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)

Answer 2

I'd suggest to check for the length outside of the regular expression, otherwise the expression might get too complicated.

Here's an example snippet in JavaScript:

if (str.length < 8 || str.length > 20)
  return false;
if (str.match(/(^\s|\s$|\s\s|[^A-Za-z.\s])/))
  return false;

The regular expression checks for a match of any of the forbidden patterns:

• ^\s a whitespace at the beginning
• \s$ a whitespace at the end
• \s\s two consecutive whitespace characters
• [^A-Za-z.\s] a character that is not a letter, period or whitespace

If you will allow only spaces (ASCII 32), not tabs or other whitespace characters, you can replace all \s by the literal space character.

Another solution would be a combination of a “positive” expression to check for the permitted characters and length and a “negative” one to rule out the rejected patterns:

return str.match(/[A-Za-z. ]){8,20}/) && !str.match(/(^ | $|  )/);

Update: If you need to put everything into a single expression, I'm afraid you have to leave out the check for consecutive spaces, because this restriction makes the language context-sensitive, hence it cannot be checked by a regular expression. What you can do is to check for a string that begins with a letter, followed by 6 to 18 letters, dots or spaces and ends with a letter:

[A-Z][A-Z. ]{6,18}[A-Z]