How to match a single space total not just a single space using regular expressions

Here is what I currently have, this will match alphanumerical characters and space:

^[a-z0-9\s]+$

What I would like to do is to ensure that there will only be a match if there is no more than one (1) space. The above will match "This is a test", but I would only want it to match if the input is "This isatest", or "T hisisatest". As soon as there is more开发者_JS百科 than one space total it will no longer match.

Ideally it would also not match if the space is leading or trailing, but that's just icing.

EDIT: The idea is that this will be used to verify account names upon account creation. The account name may only contain a Latin letter, a number, and a single space. However, it could be all letters or all numbers, the space is not required. This is definitely about space and not whitespace.

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Will allow atmost one space in between.

^[a-zA-Z0-9]+\s?[a-zA-Z0-9]+$

Will allow exactly one space in between

^[a-zA-Z0-9]+\s[a-zA-Z0-9]+$

EDIT1:

Above solutions does not accept single character strings.

The below solutions also matches single character strings

^[a-zA-Z0-9]+([ ][a-zA-Z0-9]+)?$

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First clean the username by trimming leading and trailing whitespace.

^[a-z0-9]+(?: [a-z0-9]+)?$

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Should be as simple as this. Matches 0 or 1 whitespace character in the middle of the text. Although it matches tabs too.

[a-z0-9]+\s{0,1}[a-z0-9]+

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It's fairly easy, just build it in chunks.

^[a-z0-9]+(?: [a-z0-9]+)?$

One or more alphanumerics, then an optional group consisting of a single space followed by one or more alphanumerics. (BTW, \s matches a whitespace character, not a space. That is, it actually matches [ \t\n]. Which do you actually want?)

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Try:

^[a-z0-9]+(?: [a-z0-9]+)?$

(Edited: your follow-up suggests you want to check for a space rather than for whitespace).