add chars validation

chars.go

@@ -0,0 +1,19 @@
+package validate
+
+import (
+	"fmt"
+	"strings"
+)
+
+// Chars validates whether a string contains only allowed characters.
+func Chars(allow string) func(string) error {
+	return func(value string) error {
+		rs := []rune(value)
+		for _, r := range rs {
+			if !strings.ContainsRune(allow, r) {
+				return fmt.Errorf("Contains disallowed characters")
+			}
+		}
+		return nil
+	}
+}