asp.net - Regular expression to allow only one space between words

Question

I've a textbox in an ASP.NET application, for which I need to use a regular expression to validate the user input string. Requirements for regex are -

1. It should allow only one space between words. That is, total number of spaces between words or characters should only be one.
2. It should ignore leading and trailing spaces.

Matches:

• Test
• Test abc

Non Matches:

• Test abc def
• Test abc --> I wanted to include multiple spaces between the 2 words. However the editor ignores these extra spaces while posting a question.

Answer 1

Assuming there must be either one or two 'words' (i.e. sequences of non-space characters)

"s*S+(sS+)?s*"

Change S to [A-Za-z] if you want to allow only letters.