Match trailing spaces but not lines of spaces using JavaScript RegExp

Question

I want to match trailing spaces at the end of lines, but not if the line is entirely made of whitespace.

These whitespaces should then be replaced.

The following expression uses negative lookahead to match these whitespace lines, but does not seem to work.

/(?!^( | )+$)( | )+$/gm

Here is a sample input

#The following line's trailing spaces should match
Foo     

#But not a line full of whitespace
     

And here a snippet

var oRegExp = /(?!^( | )+$)( | )+$/;

// Test cases
var test = function (sInput, sOutput) {
    var oMatch = sInput.match(oRegExp);
    var bSuccess = (oMatch === null && sOutput === null) || oMatch[0] === sOutput;
    console.log('"' + sInput + '" ', bSuccess ? 'success' : 'failed');
};
test('Foo     ', '     ');
test('     ', null);
test('', null);

Regex101 sample

Answer 1

Use regex as /\S\s+$/ which matches any non-whitespace character followed by whitespace at the end. Or more specific regex /[^ ][ ]+$/ which doesn't include newline char, tab space, etc.

Few more suggestions:

1. Actually, you are giving the argument as a regex so RegExp is completely unnecessary here.
2. The global flag doesn't have any role in your code since it only occurs once.
3. Use RegExp#test method for getting a boolean value when the string is matched.

var oRegExp = /\S\s+$/;
// or more specific
// oRegExp = /[^ ][ ]+$/;

var sValidInput = 'Foo    ';
var sInvalidInput = '    ';

console.log('"' + sValidInput + '" should match: ', oRegExp.test(sValidInput));
console.log('"' + sInvalidInput + '" should not match: ', oRegExp.test(sInvalidInput));

---

UPDATE : Use regex /^(?!(?: | )+$).*?( | )+$/ which used negative look-ahead assertion to match any other combination.

var oRegExp = /^(?!(?: | )+$).*?( | )+$/;

var sValidInput = 'Foo     ';
var sInvalidInput = '     ';


console.log('"' + sValidInput + '" should match: ', oRegExp.test(sValidInput));
console.log('"' + sInvalidInput + '" should not match: ', oRegExp.test(sInvalidInput));