regexp to allow only one space in between words

I’m trying to write a regular expression to remove white spaces from just the beginning of the word, not after, and only a single space after the word.

Used RegExp:

var re = new RegExp(/^([a-zA-Z0-9]+s?)*$/);

Test Exapmle:

1) test[space]ing - Should be allowed 
2) testing - Should be allowed 
3) [space]testing - Should not be allowed 
4) testing[space] - Should be allowed but have to trim it 
5) testing[space][space] - should be allowed but have to trim it 

Only one space should be allowed. Is it possible?

Answer

function validate(s) {
    if (/^(w+s?)*s*$/.test(s)) {
        return s.replace(/s+$/, '');
    }
    return 'NOT ALLOWED';
}
validate('test ing')    // => 'test ing'
validate('testing')     // => 'testing'
validate(' testing')    // => 'NOT ALLOWED'
validate('testing ')    // => 'testing'
validate('testing  ')   // => 'testing'
validate('test ing  ')  // => 'test ing'

BTW, new RegExp(..) is redundant if you use regular expression literal.