regex: fix find() when using anchors (start / end) (#18259)

vlib/regex/regex_anchor_test.v

@@ -0,0 +1,22 @@
+import regex
+
+fn test_anchor_start() {
+	mut re := regex.regex_opt(r'^\w+') or { panic(err) }
+	start, end := re.find('id.')
+	assert start == 0
+	assert end == 2
+}
+
+fn test_anchor_end() {
+	mut re := regex.regex_opt(r'\w+$') or { panic(err) }
+	start, end := re.find('(id')
+	assert start == 1
+	assert end == 3
+}
+
+fn test_anchor_both() {
+	mut re := regex.regex_opt(r'^\w+$') or { panic(err) }
+	start, end := re.find('(id)')
+	assert start == -1
+	assert end == -1
+}

vlib/regex/regex_util.v

@@ -202,6 +202,14 @@ pub fn (mut re RE) find(in_txt string) (int, int) {
 					re.groups[gi] += i
 					gi++
 				}
+				// when ^ (f_ms) is used, it must match on beginning of string
+				if (re.flag & f_ms) != 0 && s > 0 {
+					break
+				}
+				// when $ (f_me) is used, it must match on ending of string
+				if (re.flag & f_me) != 0 && i + e < in_txt.len {
+					break
+				}
 				return i + s, i + e
 			}
 			i++