docs: add example for ValidateFunc (#705)

* docs(textinput): add ValidateFunc example

* chore: remove print statement

* chore: rename the -> this anonymous func

textinput/textinput_test.go

@@ -1,6 +1,8 @@
 package textinput
 
 import (
+	"fmt"
+	"strconv"
 	"strings"
 	"testing"
 )
@@ -43,3 +45,36 @@ func Test_SlicingOutsideCap(t *testing.T) {
 	textinput.Width = 32
 	textinput.View()
 }
+
+func ExampleValidateFunc() {
+	creditCardNumber := New()
+	creditCardNumber.Placeholder = "4505 **** **** 1234"
+	creditCardNumber.Focus()
+	creditCardNumber.CharLimit = 20
+	creditCardNumber.Width = 30
+	creditCardNumber.Prompt = ""
+	// This anonymous function is a valid function for ValidateFunc.
+	creditCardNumber.Validate = func(s string) error {
+		// Credit Card Number should a string less than 20 digits
+		// It should include 16 integers and 3 spaces
+		if len(s) > 16+3 {
+			return fmt.Errorf("CCN is too long")
+		}
+
+		if len(s) == 0 || len(s)%5 != 0 && (s[len(s)-1] < '0' || s[len(s)-1] > '9') {
+			return fmt.Errorf("CCN is invalid")
+		}
+
+		// The last digit should be a number unless it is a multiple of 4 in which
+		// case it should be a space
+		if len(s)%5 == 0 && s[len(s)-1] != ' ' {
+			return fmt.Errorf("CCN must separate groups with spaces")
+		}
+
+		// The remaining digits should be integers
+		c := strings.ReplaceAll(s, " ", "")
+		_, err := strconv.ParseInt(c, 10, 64)
+
+		return err
+	}
+}