expr: Handle trailing backslash error

src/uu/expr/src/expr.rs

@@ -50,6 +50,8 @@ pub enum ExprError {
     UnmatchedClosingBrace,
     #[error("Invalid content of \\{{\\}}")]
     InvalidBracketContent,
+    #[error("Trailing backslash")]
+    TrailingBackslash,
 }
 
 impl UError for ExprError {

src/uu/expr/src/syntax_tree.rs

@@ -161,6 +161,7 @@ impl StringOp {
                 match first {
                     Some('^') => {} // Start of string anchor is already added
                     Some('*') => re_string.push_str(r"\*"),
+                    Some('\\') if right.len() == 1 => return Err(ExprError::TrailingBackslash),
                     Some(char) => re_string.push(char),
                     None => return Ok(0.into()),
                 };
@@ -169,6 +170,8 @@ impl StringOp {
                 let mut prev = first.unwrap_or_default();
                 let mut prev_is_escaped = false;
                 while let Some(curr) = pattern_chars.next() {
+                    let curr_is_escaped = prev == '\\' && !prev_is_escaped;
+
                     match curr {
                         '^' => match (prev, prev_is_escaped) {
                             // Start of a capturing group
@@ -201,6 +204,9 @@ impl StringOp {
                                 re_string.push('$');
                             }
                         }
+                        '\\' if !curr_is_escaped && pattern_chars.peek().is_none() => {
+                            return Err(ExprError::TrailingBackslash);
+                        }
                         _ => re_string.push(curr),
                     }
 

tests/by-util/test_expr.rs

@@ -365,6 +365,26 @@ fn test_regex() {
         .stdout_only("0\n");
 }
 
+#[test]
+fn test_regex_trailing_backslash() {
+    new_ucmd!()
+        .args(&["\\", ":", "\\\\"])
+        .succeeds()
+        .stdout_only("1\n");
+    new_ucmd!()
+        .args(&["\\", ":", "\\"])
+        .fails()
+        .stderr_only("expr: Trailing backslash\n");
+    new_ucmd!()
+        .args(&["abc\\", ":", "abc\\\\"])
+        .succeeds()
+        .stdout_only("4\n");
+    new_ucmd!()
+        .args(&["abc\\", ":", "abc\\"])
+        .fails()
+        .stderr_only("expr: Trailing backslash\n");
+}
+
 #[test]
 fn test_substr() {
     new_ucmd!()