expr: Escape anchor characters within pattern (#7842)

* expr: Escape anchor characters within the core pattern

The anchor characters `^` and `$` are not considered special characters by `expr`
unless they are used as expected on the start or end of the pattern.

src/uu/expr/src/syntax_tree.rs

@@ -150,8 +150,32 @@ impl StringOp {
                 let left = left?.eval_as_string();
                 let right = right?.eval_as_string();
                 check_posix_regex_errors(&right)?;
-                let prefix = if right.starts_with('*') { r"^\" } else { "^" };
+
-                let re_string = format!("{prefix}{right}");
+                // All patterns are anchored so they begin with a caret (^)
+                let mut re_string = String::with_capacity(right.len() + 1);
+                re_string.push('^');
+
+                // Handle first character from the input pattern
+                let mut pattern_chars = right.chars();
+                let first = pattern_chars.next();
+                match first {
+                    Some('^') => {} // Start of string anchor is already added
+                    Some('*') => re_string.push_str(r"\*"),
+                    Some(char) => re_string.push(char),
+                    None => return Ok(0.into()),
+                };
+
+                // Handle the rest of the input pattern.
+                // Escape characters that should be handled literally within the pattern.
+                let mut prev = first.unwrap_or_default();
+                for curr in pattern_chars {
+                    match curr {
+                        '^' if prev != '\\' => re_string.push_str(r"\^"),
+                        char => re_string.push(char),
+                    }
+                    prev = curr;
+                }
+
                 let re = Regex::with_options(
                     &re_string,
                     RegexOptions::REGEX_OPTION_NONE,

tests/by-util/test_expr.rs

@@ -274,11 +274,10 @@ fn test_length_mb() {
 
 #[test]
 fn test_regex() {
-    // FixME: [2022-12-19; rivy] test disabled as it currently fails due to 'oniguruma' bug (see GH:kkos/oniguruma/issues/279)
+    new_ucmd!()
-    // new_ucmd!()
+        .args(&["a^b", ":", "a^b"])
-    //     .args(&["a^b", ":", "a^b"])
+        .succeeds()
-    //     .succeeds()
+        .stdout_only("3\n");
-    //     .stdout_only("3\n");
     new_ucmd!()
         .args(&["a^b", ":", "a\\^b"])
         .succeeds()
@@ -288,13 +287,38 @@ fn test_regex() {
         .succeeds()
         .stdout_only("3\n");
     new_ucmd!()
-        .args(&["-5", ":", "-\\{0,1\\}[0-9]*$"])
+        .args(&["abc", ":", "^abc"])
+        .succeeds()
+        .stdout_only("3\n");
+    new_ucmd!()
+        .args(&["^abc", ":", "^^abc"])
+        .succeeds()
+        .stdout_only("4\n");
+    new_ucmd!()
+        .args(&["b^$ic", ":", "b^\\$ic"])
+        .succeeds()
+        .stdout_only("5\n");
+    new_ucmd!()
+        .args(&["^^^^^^^^^", ":", "^^^"])
         .succeeds()
         .stdout_only("2\n");
+    new_ucmd!()
+        .args(&["-5", ":", "-\\{0,1\\}[0-9]*$"])
+        .succeeds()
+        .stdout_only("2\n");
+    new_ucmd!().args(&["", ":", ""]).fails().stdout_only("0\n");
+    new_ucmd!()
+        .args(&["abc", ":", ""])
+        .fails()
+        .stdout_only("0\n");
     new_ucmd!()
         .args(&["abc", ":", "bc"])
         .fails()
         .stdout_only("0\n");
+    new_ucmd!()
+        .args(&["^abc", ":", "^abc"])
+        .fails()
+        .stdout_only("0\n");
 }
 
 #[test]
@@ -711,7 +735,6 @@ mod gnu_expr {
             .stdout_only("\n");
     }
 
-    #[ignore = "rust-onig bug, see https://github.com/rust-onig/rust-onig/issues/188"]
     #[test]
     fn test_bre10() {
         new_ucmd!()