Using Symfony\Component\ExpressionLanguage::tokenize() with ' ' (only spaces) or any expression with trailing spaces like '1 + 1 ' will produce an Uninitialized string offset: X php warning due to the way spaces are ignored:

// Symfony\Component\ExpressionLanguage::tokenize() lines 39 to 41

while (' ' == $expression[$cursor]) {
    ++$cursor;
}

If EOF_TYPE Token position is not a problem, a rtrim on passed expression will do the job.