这是一篇很老的帖子，但也许有人会发现这个解决方案很有用。

不幸的是，搜索文本并确定其在 RichTextBox 中的位置是一个有点复杂的过程。由于文本可以包含不同的格式元素，例如背景/前景色、不同的字体、图像等，因此必须使用上下文来确定所需文本的位置。

一种可能的解决方案如下所示。

MainWindow.xaml

<Window ...
        Title="MainWindow" Height="250" Width="400"
        KeyDown="Window_KeyDown">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="*"/>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>
        <RichTextBox Grid.Row="0" x:Name="rtb" AllowDrop="True" 
                     VerticalScrollBarVisibility="Auto" Padding="2" FontSize="18" 
                     HorizontalAlignment="Left" >
            <FlowDocument>
                <Paragraph>
                    <Run FontSize="10"  Text="In this example,you will learn to print Hello World." />
                    <Run Text="In this example,you will learn to print Hello World." />
                    <Run FontSize="20" Text="In this example,you will learn to print Hello World." />                   
                </Paragraph>
            </FlowDocument>
        </RichTextBox>

        <Button Grid.Row="1" Click="Button_Click">Color Words</Button>
    </Grid>
</Window> 

MainWindow.xaml.cs

private void Button_Click(object sender,RoutedEventArgs e)
{
    var pairs = new Dictionary<string,SolidColorBrush>(StringComparer.InvariantCultureIgnoreCase)
    {
        { "hello",Brushes.Red },{ "world",Brushes.Blue }
    };

    // Define the range to be used to analyze for the specified words
    var textRange = new TextRange(rtb.Document.ContentStart,rtb.Document.ContentEnd);

    // Build list of Word/TextRange pairs
    var list = CalculateTextRange(textRange,pairs.Select(d => d.Key).ToList());

    // Color words by using calculated `TextRange`.
    for (int i = 0; i < list.Count; i++)
    {
        list[i].Value.ApplyPropertyValue(TextElement.ForegroundProperty,pairs[list[i].Key]);
    }

    rtb.Focus();
}

public static IList<KeyValuePair<string,TextRange>> CalculateTextRange(TextRange range,IList<string> words)
{
    // Building the regular expression pattern 
    var pattern = new StringBuilder(@"\b");
    for (int i = 0; i < words.Count; i++)
    {
        if (i > 0) pattern.Append('|');
        pattern.Append(words[i]);
    }
    pattern.Append(@"\b");
    Regex regExp = new Regex(pattern.ToString(),RegexOptions.IgnoreCase);

    TextRange search = range;
    int correction = 0;
    var result = new List<KeyValuePair<string,TextRange>>();

    // Enumerate all found mathes and creating list of Word/TextRange pairs
    foreach (Match match in regExp.Matches(range.Text))
    {
        if (CalculateTextRange(search,match.Index - correction,match.Length) is TextRange tr)
        {
            result.Add(new KeyValuePair<string,TextRange>(match.Value,tr));
            correction = match.Index + match.Length;
            search = new TextRange(tr.End,search.End);
        }
    }
    return result;
}
        
// Calculates a `TextRange` of the string started from `iStart` index
// and having `length` size or `null`.
public static TextRange CalculateTextRange(TextRange search,int iStart,int length)
{
    return (GetTextPositionAtOffset(search.Start,iStart) is TextPointer start)
        ? new TextRange(start,GetTextPositionAtOffset(start,length))
        : null;
}

// Calculate `TextPointer` from defined text position by specified offset.
public static TextPointer GetTextPositionAtOffset(TextPointer position,int offset)
{
    for (TextPointer current = position; current != null; current = position.GetNextContextPosition(LogicalDirection.Forward))
    {
        position = current;
        var adjacent = position.GetAdjacentElement(LogicalDirection.Forward);
        var context = position.GetPointerContext(LogicalDirection.Forward);
        switch (context)
        {
            case TextPointerContext.Text:
                int count = position.GetTextRunLength(LogicalDirection.Forward);
                if (offset <= count)
                {
                    return position.GetPositionAtOffset(offset);
                }
                offset -= count;
                break;
            case TextPointerContext.ElementStart:
                if (adjacent is InlineUIContainer)
                    offset--;
                break;
            case TextPointerContext.ElementEnd:
                if (adjacent is Paragraph)
                    offset -= 2;
                break;
        }
    }
    return position;
}

上面的代码包含 Button_Click() 处理程序，当用户点击 Color Words 按钮时，它会执行以下步骤。

1. 定义要在 RichTextBox 中找到的单词列表和列表中每个单词的颜色（在本例中实际上是画笔）：

var pairs = new Dictionary<string,Brushes.Blue }
    }; 

2. 定义用于分析指定词的范围。在此示例中，使用了整个 RichBoxText 文档：

var textRange = new TextRange(rtb.Document.ContentStart,rtb.Document.ContentEnd);

3. 为列表中指定并在文档中找到的所有单词计算 TextRange 的列表：

// Build list of Word/TextRange pairs
var list = CalculateTextRange(textRange,pairs.Select(d => d.Key).ToList());

4. 为所有找到的单词着色。

添加图片是为了检查即使在这种情况下也能正确计算位置。