Question

I have a string like this one text more text "empty space".
How can I replace the space in "empty space" and only this space with ###?

Answer 1

How about this, with no regular expressions:

$text = 'foo bar "baz quux"';
$parts = explode('"', $text);
$inQuote = false;

foreach ($parts as &$part) {
    if ($inQuote) { $part = str_replace(' ', '###', $part); }
    $inQuote = !$inQuote;
}

$parsed = implode('"', $parts);
echo $parsed;