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Regex [\S but not [[\S?
Aka:  This is ok:  [s
      This should be skipped:  [[s

foreach (Rule rule in rules)
{
    if (position + rule.Length > text.Length)
        break;
    if ((rule.PreCondition & condition) == rule.PreCondition)
    {
        if (substring.Length < rule.Length)
            substring = text.Substring(position, rule.Length);
        if (rule.Pattern.IsMatch(substring))
        {
            string previous = text.Substring(0, position);
            sb.Append(previous);
            text = text.Remove(0, position);
            position = 0;
            text = rule.Handler(new HandlerParameter(text, condition, previous));
        }
    }
}

@"(?<![)[[^[s]"

@"(^|[^[])[[^[s]"

@"[[^s[]"