fix(bash): BASH_REMATCH not populated when regex starts with parens (#242)

## Summary
- Fix parser's parse_conditional to check saw_regex_op flag when
encountering LeftParen token inside [[ ]]
- Regex patterns starting with capture groups (e.g. ([0-9]+)) were
incorrectly parsed as grouping parens instead of regex content
- Adds 6 test cases covering basic match, multiple groups, nested
groups, no-match clearing, and conditional usage

## Test plan
- [x] All 6 new regex test cases pass
- [x] cargo test --all-features passes
- [x] bash_comparison_tests pass (validated against real bash)
- [x] cargo clippy and cargo fmt clean

Co-authored-by: Claude <noreply@anthropic.com>

Author: chaliy

crates/bashkit/src/parser/mod.rs

@@ -1078,6 +1078,13 @@ impl<'a> Parser<'a> {
                     self.advance();
                 }
                 Some(tokens::Token::LeftParen) => {
+                    if saw_regex_op {
+                        // Regex pattern starts with '(' — collect it
+                        let pattern = self.collect_conditional_regex_pattern("(");
+                        words.push(Word::literal(&pattern));
+                        saw_regex_op = false;
+                        continue;
+                    }
                     words.push(Word::literal("("));
                     self.advance();
                 }

crates/bashkit/tests/spec_cases/bash/control-flow.test.sh

@@ -271,3 +271,72 @@ ERR
 done
 BYE
 ### end
+
+### regex_match_basic
+# [[ =~ ]] regex match returns correct exit code
+[[ "hello123" =~ [0-9]+ ]]; echo $?
+[[ "hello" =~ [0-9]+ ]]; echo $?
+### expect
+0
+1
+### end
+
+### regex_match_bash_rematch
+# BASH_REMATCH populated with capture groups
+x="hello123world"
+[[ "$x" =~ ([0-9]+) ]]
+echo "${BASH_REMATCH[0]}"
+echo "${BASH_REMATCH[1]}"
+### expect
+123
+123
+### end
+
+### regex_match_multiple_groups
+# Multiple capture groups in BASH_REMATCH
+[[ "2024-01-15" =~ ^([0-9]{4})-([0-9]{2})-([0-9]{2})$ ]]
+echo "${BASH_REMATCH[0]}"
+echo "${BASH_REMATCH[1]}"
+echo "${BASH_REMATCH[2]}"
+echo "${BASH_REMATCH[3]}"
+### expect
+2024-01-15
+2024
+01
+15
+### end
+
+### regex_match_nested_groups
+# Nested capture groups
+[[ "foo123bar" =~ (foo([0-9]+)bar) ]]
+echo "${BASH_REMATCH[0]}"
+echo "${BASH_REMATCH[1]}"
+echo "${BASH_REMATCH[2]}"
+### expect
+foo123bar
+foo123bar
+123
+### end
+
+### regex_match_no_match_clears
+# BASH_REMATCH cleared on no match
+[[ "abc123" =~ ([0-9]+) ]]
+echo "before: ${#BASH_REMATCH[@]}"
+[[ "abc" =~ ([0-9]+) ]]
+echo "after: ${#BASH_REMATCH[@]}"
+### expect
+before: 2
+after: 0
+### end
+
+### regex_match_in_conditional
+# Regex match used in && chain
+x="error: line 42"
+if [[ "$x" =~ error:\ line\ ([0-9]+) ]]; then
+  echo "line ${BASH_REMATCH[1]}"
+else
+  echo "no match"
+fi
+### expect
+line 42
+### end

specs/009-implementation-status.md

@@ -103,16 +103,16 @@ Bashkit implements IEEE 1003.1-2024 Shell Command Language. See
 
 ## Spec Test Coverage
 
-**Total spec test cases:** 1161 (1156 pass, 5 skip)
+**Total spec test cases:** 1170 (1165 pass, 5 skip)
 
 | Category | Cases | In CI | Pass | Skip | Notes |
 |----------|-------|-------|------|------|-------|
-| Bash (core) | 800 | Yes | 795 | 5 | `bash_spec_tests` in CI |
+| Bash (core) | 809 | Yes | 804 | 5 | `bash_spec_tests` in CI |
 | AWK | 96 | Yes | 96 | 0 | loops, arrays, -v, ternary, field assign, getline, %.6g |
 | Grep | 76 | Yes | 76 | 0 | -z, -r, -a, -b, -H, -h, -f, -P, --include, --exclude, binary detect |
 | Sed | 75 | Yes | 75 | 0 | hold space, change, regex ranges, -E |
 | JQ | 114 | Yes | 114 | 0 | reduce, walk, regex funcs, --arg/--argjson, combined flags, input/inputs, env |
-| **Total** | **1161** | **Yes** | **1156** | **5** | |
+| **Total** | **1170** | **Yes** | **1165** | **5** | |
 
 ### Bash Spec Tests Breakdown
 
@@ -128,15 +128,15 @@ Bashkit implements IEEE 1003.1-2024 Shell Command Language. See
 | command-not-found.test.sh | 17 | unknown command handling |
 | conditional.test.sh | 17 | `[[ ]]` conditionals, `=~` regex, BASH_REMATCH |
 | command-subst.test.sh | 14 | includes backtick substitution (1 skipped) |
-| control-flow.test.sh | 37 | if/elif/else, for, while, case, trap ERR |
+| control-flow.test.sh | 43 | if/elif/else, for, while, case, trap ERR, `[[ =~ ]]` BASH_REMATCH |
 | cuttr.test.sh | 32 | cut and tr commands, `-z` zero-terminated |
 | date.test.sh | 38 | format specifiers, `-d` relative/compound/epoch, `-R`, `-I`, `%N` (2 skipped) |
 | diff.test.sh | 4 | line diffs |
 | echo.test.sh | 24 | escape sequences |
 | errexit.test.sh | 8 | set -e tests |
 | fileops.test.sh | 21 | |
 | find.test.sh | 10 | file search |
-| functions.test.sh | 14 | |
+| functions.test.sh | 17 | local dynamic scoping, nested writes |
 | getopts.test.sh | 9 | POSIX option parsing, combined flags, silent mode |
 | globs.test.sh | 12 | for-loop glob expansion, recursive `**` |
 | headtail.test.sh | 14 | |