见代码如下：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApp1
{
    public partial class FormFYSD : Form
    {
        public FormFYSD()
        {
            InitializeComponent();
        }

        private void textBox3_KeyPress(object sender, KeyPressEventArgs e)
        {
            bool result = false;
            //判断当前textBox是否为空
            if (!string.IsNullOrEmpty(textBox3.Text.Trim()))
            {
                //Regex rex = new Regex(@"^[0-9]*$");
                Regex rexFull = new Regex(@"^[0-9]+(.[0-9]{0,1})?$");
                //判断输入是否是退格键
                if (e.KeyChar == '\b')
                {
                    result = false;
                }
                else
                {
                    //判断是否匹配正则表达式
                    if (rexFull.IsMatch(textBox3.Text.Trim()) || rexFull.IsMatch(textBox3.Text.Trim() + e.KeyChar.ToString()))
                    {
                        //判断字符串中小数点的位数
                        if (Regex.Matches(textBox3.Text.Trim() + e.KeyChar.ToString(), "\\.").Count == 2)
                        {
                            result = true;
                        }
                        else
                        {
                            //判断输入字符是否是数字或者小数点
                            if (!(char.IsNumber(e.KeyChar) || e.KeyChar == (char)('.')))
                            {
                                result = true;
                            }
                            else
                            {
                                result = false;
                            }
                        }
                    }
                    else
                    {
                        result = true;
                    }
                }
            }
            else
            {
                if (e.KeyChar < '0' || e.KeyChar > '9')//这是不允许输入0-9数字
                {
                    result = true;
                }
                else
                {
                    result = false;
                }
            }
            e.Handled = result;
        }
    }
}

这里需要注意添加一个引用：using System.Text.RegularExpressions;否则Regex无法识别。

 

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