fix(parser): expand variables in [[ =~ $var ]] regex patterns

When regex pattern after =~ operator contains a variable reference ($),
use parse_word() for proper expansion instead of collect_conditional_regex_pattern()
which creates literal words. Literal regex patterns (without $) still use
the special collector to preserve parens, backslashes, etc.

Closes #400

https://claude.ai/code/session_01WZjYqxm5xMPAEe7FSHJkDy

Author: claude

crates/bashkit/src/interpreter/mod.rs

@@ -9686,4 +9686,26 @@ bash /tmp/opts.sh -f xml -v
         assert_eq!(lines[0], "FORMAT=json VERBOSE=1");
         assert_eq!(lines[1], "FORMAT=xml VERBOSE=1");
     }
+
+    #[tokio::test]
+    async fn test_regex_match_from_variable() {
+        // Issue #400: [[ =~ $var ]] should work with regex from variable
+        let mut bash = crate::Bash::new();
+        let result = bash
+            .exec(r#"re="200"; line="hello 200 world"; [[ $line =~ $re ]] && echo "match" || echo "no""#)
+            .await
+            .unwrap();
+        assert_eq!(result.stdout.trim(), "match");
+    }
+
+    #[tokio::test]
+    async fn test_regex_match_literal() {
+        // Issue #400: literal regex should still work
+        let mut bash = crate::Bash::new();
+        let result = bash
+            .exec(r#"line="hello 200 world"; [[ $line =~ 200 ]] && echo "match" || echo "no""#)
+            .await
+            .unwrap();
+        assert_eq!(result.stdout.trim(), "match");
+    }
 }

crates/bashkit/src/parser/mod.rs

@@ -1263,10 +1263,20 @@ impl<'a> Parser<'a> {
                     let is_literal =
                         matches!(self.current_token, Some(tokens::Token::LiteralWord(_)));
 
-                    // After =~, collect the regex pattern (may contain parens)
+                    // After =~, handle regex pattern.
+                    // If the pattern contains $ (variable reference), parse it as a
+                    // normal word so variables expand. Otherwise collect as literal
+                    // regex to preserve parens, backslashes, etc.
                     if saw_regex_op {
-                        let pattern = self.collect_conditional_regex_pattern(&w_clone);
-                        words.push(Word::literal(&pattern));
+                        if w_clone.contains('$') && !is_quoted {
+                            // Variable reference — parse normally for expansion
+                            let parsed = self.parse_word(w_clone);
+                            words.push(parsed);
+                            self.advance();
+                        } else {
+                            let pattern = self.collect_conditional_regex_pattern(&w_clone);
+                            words.push(Word::literal(&pattern));
+                        }
                         saw_regex_op = false;
                         continue;
                     }