expr: Refactor checking if '$' is the last character

Author: frendsick

src/uu/expr/src/syntax_tree.rs

@@ -193,9 +193,9 @@ impl StringOp {
                                     _ => re_string.push_str(r"\$"),
                                 }
                                 re_string.push(backslash);
-                            } else if pattern_chars.peek().is_none() {
-                                re_string.push('$');
-                            } else if prev_is_escaped || prev != '\\' {
+                            } else if (prev_is_escaped || prev != '\\')
+                                && pattern_chars.peek().is_some()
+                            {
                                 re_string.push_str(r"\$");
                             } else {
                                 re_string.push('$');