builtin: add a contains_only method on string (#14830)

vlib/builtin/string.v

@@ -1130,6 +1130,23 @@ pub fn (s string) contains_any(chars string) bool {
 	return false
 }
 
+// contains_only returns `true`, if the string contains only the characters in `chars`.
+pub fn (s string) contains_only(chars string) bool {
+	if chars.len == 0 {
+		return false
+	}
+	for ch in s {
+		mut res := 0
+		for i := 0; i < chars.len && res == 0; i++ {
+			res += int(ch == unsafe { chars.str[i] })
+		}
+		if res == 0 {
+			return false
+		}
+	}
+	return true
+}
+
 // contains_any_substr returns `true` if the string contains any of the strings in `substrs`.
 pub fn (s string) contains_any_substr(substrs []string) bool {
 	if substrs.len == 0 {

vlib/builtin/string_test.v

@@ -447,6 +447,13 @@ fn test_contains_any() {
 	assert !''.contains_any('')
 }
 
+fn test_contains_only() {
+	assert '23885'.contains_only('0123456789')
+	assert '23gg885'.contains_only('01g23456789')
+	assert !'hello;'.contains_only('hello')
+	assert !''.contains_only('')
+}
+
 fn test_contains_any_substr() {
 	s := 'Some random text'
 	assert s.contains_any_substr(['false', 'not', 'rand'])