RichTextBox双击选择由句点分隔的单词，如“i.a”。

using System;
using System.Windows.Forms;

public class CustomRichTextBox : RichTextBox
{
    protected override void OnMouseDoubleClick(MouseEventArgs e)
    {
        int pos = GetPositionFromCharIndex(Cursor.Position.X);
        string text = this.Text;
        int start = pos;
        int end = pos;

        // 向左查找单词边界
        while (start > 0 && !IsWordBoundary(text[start - 1]))
        {
            start--;
        }

        // 向右查找单词边界
        while (end < text.Length && !IsWordBoundary(text[end]))
        {
            end++;
        }

        this.SelectionStart = start;
        this.SelectionLength = end - start;
    }

    private bool IsWordBoundary(char c)
    {
        return char.IsWhiteSpace(c) || c == '.' || c == ',' || c == ';' || c == ':' || c == '!' || c == '?' || c == '(' || c == ')' || c == '[' || c == ']' || c == '{' || c == '}' || c == '"' || c == '\'';
    }
}