encoding.utf8: add `pub fn is_letter(r rune) bool` (#11547)

vlib/encoding/utf8/utf8_util.v

@@ -141,6 +141,25 @@ pub fn is_punct(s string, index int) bool {
 	return is_uchar_punct(get_uchar(s, index))
 }
 
+// is_control return true if the rune is control code
+pub fn is_control(r rune) bool {
+	// control codes are all below 0xff
+	if r > max_latin_1 {
+		return false
+	}
+	return props[byte(r)] == 1
+}
+
+// is_letter returns true if the rune is unicode letter or in unicode category L
+pub fn is_letter(r rune) bool {
+	if (r >= `a` && r <= `z`) || (r >= `A` && r <= `Z`) {
+		return true
+	} else if r <= max_latin_1 {
+		return props[byte(r)] & p_l_mask != 0
+	}
+	return is_excluding_latin(letter_table, r)
+}
+
 // is_uchar_punct return true if the input unicode is a western unicode punctuation
 pub fn is_uchar_punct(uchar int) bool {
 	return find_punct_in_table(uchar, utf8.unicode_punct_western) != 0

vlib/encoding/utf8/utf8_util_test.v

@@ -64,3 +64,30 @@ fn test_reversed() {
 	assert utf8.reverse(a) == '!gnaL V是我'
 	assert utf8.reverse(b) == 'dlrow olleh界世好你'
 }
+
+fn test_is_control() {
+	for ra in `a` .. `z` {
+		assert utf8.is_control(ra) == false
+	}
+
+	for ra in `A` .. `Z` {
+		assert utf8.is_control(ra) == false
+	}
+
+	assert utf8.is_control('\x01'.runes()[0]) == true
+	assert utf8.is_control('\u0100'.runes()[0]) == false
+}
+
+fn test_is_letter() {
+	for ra in `a` .. `z` {
+		assert utf8.is_letter(ra) == true
+	}
+
+	for ra in `A` .. `Z` {
+		assert utf8.is_letter(ra) == true
+	}
+
+	assert utf8.is_letter(`ɀ`) == true
+	assert utf8.is_letter(`ȶ`) == true
+	assert utf8.is_letter(`ȹ`) == true
+}