keywords=>cat,dog
text=> a catchy cat with a dogged dog

var results= keywords.Select(x=>
                               new
                               {
                                word=x,
                                indexes=Regex.Matches(input,@"b"+x+@"b")
                                             .Cast<Match>().Select(y=>y.Index)
                                             .ToList()
                               }
                            );

foreach(var match in results)
{
    match.word;
    foreach(int index in match.indexes)//index
}

private List<int> GetIndexForKeyWord(string content,string key)
{
    int index = 0;
    List<int> indexes=new List<int>();
    while (index < content.Length && index >= 0)
    {
        index = content.IndexOf(key, index);
        if (index+key.Length==content.Length||index >= 0 && !char.IsLetter(content[index + key.Length]))
        {
            indexes.Add(index);
        }
        if(index!=-1)
            index++;
    }
    return indexes;
}