Regex that accepts only numbers (0-9) and NO characters
In regex ^[0-9] matches anything beginning with a digit, including strings like “1A”. To avoid a partial match, append a $ to the end:
Above code accepts any number of digits, including none. To accept one or more digits, change the * to +. To accept exactly one digit, remove the *.
Mixed up the arguments to IsMatch. The pattern is the second argument, not the first:
if (!System.Text.RegularExpressions.Regex.IsMatch(textbox.Text, "^[0-9]*$"))