regex: define the predicate matches_string (#11422)

vlib/regex/regex_test.v

@@ -556,6 +556,13 @@ fn test_regex(){
 			continue
 		}
 
+		// test the match predicate
+		if to.s >= 0 {
+			assert re.matches_string(to.src)
+		} else {
+			assert !re.matches_string(to.src)
+		}
+
 		// rerun to test consistency
 		tmp_str1 := to.src.clone()
 		start1, end1 := re.match_string(tmp_str1)

vlib/regex/regex_util.v

@@ -144,6 +144,12 @@ pub fn (mut re RE) match_string(in_txt string) (int, int) {
 	return start, end
 }
 
+// matches_string Checks if the pattern matches the in_txt string
+pub fn (mut re RE) matches_string(in_txt string) bool {
+	start, _ := re.match_string(in_txt)
+	return start != no_match_found
+}
+
 /******************************************************************************
 *
 * Finders