builtin: add string.is_identifier() (#24350)

vlib/builtin/string.v

@@ -2768,7 +2768,7 @@ pub fn (name string) match_glob(pattern string) bool {
 }
 
 // is_ascii returns true if all characters belong to the US-ASCII set ([` `..`~`])
-@[inline]
+@[direct_array_access; inline]
 pub fn (s string) is_ascii() bool {
 	for i := 0; i < s.len; i++ {
 		if s[i] < u8(` `) || s[i] > u8(`~`) {
@@ -2778,6 +2778,24 @@ pub fn (s string) is_ascii() bool {
 	return true
 }
 
+// is_identifier checks if a string is a valid identifier (starts with letter/underscore, followed by letters, digits, or underscores)
+@[direct_array_access]
+pub fn (s string) is_identifier() bool {
+	if s.len == 0 {
+		return false
+	}
+	if !(s[0].is_letter() || s[0] == `_`) {
+		return false
+	}
+	for i := 1; i < s.len; i++ {
+		c := s[i]
+		if !(c.is_letter() || c.is_digit() || c == `_`) {
+			return false
+		}
+	}
+	return true
+}
+
 // camel_to_snake convert string from camelCase to snake_case
 // Example: assert 'Abcd'.camel_to_snake() == 'abcd'
 // Example: assert 'aaBB'.camel_to_snake() == 'aa_bb'

vlib/builtin/string_test.v

@@ -1553,6 +1553,20 @@ fn test_string_is_ascii() {
 	assert 'a👋bc'.is_ascii() == false
 }
 
+fn test_string_is_identifier() {
+	assert ''.is_identifier() == false
+	assert ' '.is_identifier() == false
+	assert '~~'.is_identifier() == false
+	assert '_Az~'.is_identifier() == false
+	assert '_Aö~'.is_identifier() == false
+	assert '👋'.is_identifier() == false
+	assert 'a👋bc'.is_identifier() == false
+	assert '9'.is_identifier() == false
+	assert '_9'.is_identifier() == true
+	assert 'a 9'.is_identifier() == false
+	assert 't'.is_identifier() == true
+}
+
 fn test_string_with_zero_byte_escape() {
 	assert '\x00'.bytes() == [u8(0)]
 }