newData = Regex.Replace(textData, "(" + String.Join("|", keys) + ")",
                        new MatchEvaluator(this.EvaluateReplacement));

string EvaluateReplacement(Match m)
{
    if (this.convertDict.ContainsKey(m.Value))
    {
        return this.convertDict[m.Value];
    }
    return m.Value;
}