expr: Handle caret '^' at the beginning of an alternative pattern

Author: frendsick

src/uu/expr/src/syntax_tree.rs

@@ -174,6 +174,8 @@ impl StringOp {
                         '^' => match (prev, prev_is_escaped) {
                             // Start of a capturing group
                             ('(', true) => re_string.push(curr),
+                            // Start of an alternative pattern
+                            ('|', true) => re_string.push(curr),
                             // Character class negation "[^a]"
                             ('[', false) => re_string.push(curr),
                             // Explicitly escaped caret

tests/by-util/test_expr.rs

@@ -282,6 +282,10 @@ 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(&["a$b", ":", "a\\$b"])
         .succeeds()