expr: Fix parsing regex anchors '^' and '$'

src/uu/expr/src/syntax_tree.rs

@@ -156,7 +156,7 @@ impl StringOp {
                 re_string.push('^');
 
                 // Handle first character from the input pattern
-                let mut pattern_chars = right.chars();
+                let mut pattern_chars = right.chars().peekable();
                 let first = pattern_chars.next();
                 match first {
                     Some('^') => {} // Start of string anchor is already added
@@ -169,13 +169,39 @@ impl StringOp {
                 // Escaped previous character should not affect the current.
                 let mut prev = first.unwrap_or_default();
                 let mut prev_is_escaped = false;
-                for curr in pattern_chars {
+                while let Some(curr) = pattern_chars.next() {
                     match curr {
-                        // Carets are interpreted literally, unless used as character class negation "[^a]"
-                        '^' if prev_is_escaped || !matches!(prev, '\\' | '[') => {
-                            re_string.push_str(r"\^");
+                        '^' => match (prev, prev_is_escaped) {
+                            // Start of a capturing group
+                            ('(', true)
+                            // Start of an alternative pattern
+                            | ('|', true)
+                            // Character class negation "[^a]"
+                            | ('[', false)
+                            // Explicitly escaped caret
+                            | ('\\', false) => re_string.push(curr),
+                            _ => re_string.push_str(r"\^"),
+                        },
+                        '$' => {
+                            if let Some('\\') = pattern_chars.peek() {
+                                let backslash = pattern_chars.next().unwrap_or_default();
+                                match pattern_chars.peek() {
+                                    // End of a capturing group
+                                    Some(')') => re_string.push('$'),
+                                    // End of an alternative pattern
+                                    Some('|') => re_string.push('$'),
+                                    _ => re_string.push_str(r"\$"),
+                                }
+                                re_string.push(backslash);
+                            } else if (prev_is_escaped || prev != '\\')
+                                && pattern_chars.peek().is_some()
+                            {
+                                re_string.push_str(r"\$");
+                            } else {
+                                re_string.push('$');
+                            }
                         }
-                        char => re_string.push(char),
+                        _ => re_string.push(curr),
                     }
 
                     prev_is_escaped = prev == '\\' && !prev_is_escaped;

tests/by-util/test_expr.rs

@@ -282,10 +282,26 @@ fn test_regex() {
         .args(&["a^b", ":", "a\\^b"])
         .succeeds()
         .stdout_only("3\n");
+    new_ucmd!()
+        .args(&["b", ":", "a\\|^b"])
+        .succeeds()
+        .stdout_only("1\n");
+    new_ucmd!()
+        .args(&["ab", ":", "\\(^a\\)b"])
+        .succeeds()
+        .stdout_only("a\n");
     new_ucmd!()
         .args(&["a$b", ":", "a\\$b"])
         .succeeds()
         .stdout_only("3\n");
+    new_ucmd!()
+        .args(&["a", ":", "a$\\|b"])
+        .succeeds()
+        .stdout_only("1\n");
+    new_ucmd!()
+        .args(&["ab", ":", "a\\(b$\\)"])
+        .succeeds()
+        .stdout_only("b\n");
     new_ucmd!()
         .args(&["abc", ":", "^abc"])
         .succeeds()
@@ -766,7 +782,6 @@ mod gnu_expr {
             .stdout_only("3\n");
     }
 
-    #[ignore]
     #[test]
     fn test_bre11() {
         new_ucmd!()