diff --git a/tests/test_bcftools_validation.py b/tests/test_bcftools_validation.py
index 2ed90e8..a3d742f 100644
--- a/tests/test_bcftools_validation.py
+++ b/tests/test_bcftools_validation.py
@@ -127,7 +127,9 @@ def test_vcf_output_with_output_option(tmp_path, args, vcf_file):
         (r"query -f '%FILTER\n'", "sample.vcf.gz"),
         (r"query --format '%FILTER\n'", "1kg_2020_chrM.vcf.gz"),
         (r"query -f '%POS\n' -i 'POS=112'", "sample.vcf.gz"),
-        (r"query -f '%POS\n' -e 'POS=112'", "sample.vcf.gz")
+        (r"query -f '%POS\n' -e 'POS=112'", "sample.vcf.gz"),
+        (r"query -f '[%CHROM\t]\n'", "sample.vcf.gz"),
+        (r"query -f '[%CHROM\t]\n' -i 'POS=112'", "sample.vcf.gz"),
     ],
 )
 def test_output(tmp_path, args, vcf_name):
diff --git a/vcztools/query.py b/vcztools/query.py
index 6ad0a09..a93dbb1 100644
--- a/vcztools/query.py
+++ b/vcztools/query.py
@@ -27,21 +27,21 @@ def __init__(self):
         tag_pattern = info_tag_pattern | pp.Combine(
             pp.Literal("%") + pp.Regex(r"[A-Z]+\d?")
         )
-        subscript_pattern = pp.Group(
+        subfield_pattern = pp.Group(
             tag_pattern
             + pp.Literal("{").suppress()
             + pp.common.integer
             + pp.Literal("}").suppress()
-        )
+        ).set_results_name("subfield")
         newline_pattern = pp.Literal("\\n").set_parse_action(pp.replace_with("\n"))
         tab_pattern = pp.Literal("\\t").set_parse_action(pp.replace_with("\t"))
         format_pattern = pp.Forward()
         sample_loop_pattern = pp.Group(
             pp.Literal("[").suppress() + format_pattern + pp.Literal("]").suppress()
-        )
+        ).set_results_name("sample loop")
         format_pattern <<= pp.ZeroOrMore(
             sample_loop_pattern
-            | subscript_pattern
+            | subfield_pattern
             | tag_pattern
             | newline_pattern
             | tab_pattern
@@ -83,7 +83,9 @@ def __call__(self, *args, **kwargs):
 
         yield from self._generator(args[0])
 
-    def _compose_tag_generator(self, tag: str, *, subscript=False) -> Callable:
+    def _compose_tag_generator(
+        self, tag: str, *, subfield=False, sample_loop=False
+    ) -> Callable:
         assert tag.startswith("%")
         tag = tag[1:]
 
@@ -122,20 +124,26 @@ def generate(root):
                             row = "."
                         else:
                             row = f"{row:g}"
-                    if not subscript and (
+                    if not subfield and (
                         isinstance(row, np.ndarray) or isinstance(row, list)
                     ):
                         row = ",".join(map(str, row))
 
-                    yield row if not is_missing else "."
+                    result = row if not is_missing else "."
+
+                    if sample_loop:
+                        sample_count = root["sample_id"].shape[0]
+                        yield itertools.repeat(row, sample_count)
+                    else:
+                        yield result
 
         return generate
 
-    def _compose_subscript_generator(self, parse_results: pp.ParseResults) -> Callable:
+    def _compose_subfield_generator(self, parse_results: pp.ParseResults) -> Callable:
         assert len(parse_results) == 2
 
-        tag, subscript_index = parse_results
-        tag_generator = self._compose_tag_generator(tag, subscript=True)
+        tag, subfield_index = parse_results
+        tag_generator = self._compose_tag_generator(tag, subfield=True)
 
         def generate(root):
             for tag in tag_generator(root):
@@ -143,28 +151,61 @@ def generate(root):
                     assert tag == "."
                     yield "."
                 else:
-                    if subscript_index < len(tag):
-                        yield tag[subscript_index]
+                    if subfield_index < len(tag):
+                        yield tag[subfield_index]
                     else:
                         yield "."
 
         return generate
 
+    def _compose_sample_loop_generator(
+        self, parse_results: pp.ParseResults
+    ) -> Callable:
+        generators = map(
+            functools.partial(self._compose_element_generator, sample_loop=True),
+            parse_results,
+        )
+
+        def generate(root):
+            iterables = (generator(root) for generator in generators)
+            zipped = zip(*iterables)
+            zipped_zipped = (zip(*element) for element in zipped)
+            flattened_zipped_zipped = (
+                (
+                    subsubelement
+                    for subelement in element  # sample-wise
+                    for subsubelement in subelement
+                )
+                for element in zipped_zipped  # variant-wise
+            )
+            yield from map("".join, flattened_zipped_zipped)
+
+        return generate
+
     def _compose_element_generator(
-        self, element: Union[str, pp.ParseResults]
+        self, element: Union[str, pp.ParseResults], *, sample_loop=False
     ) -> Callable:
         if isinstance(element, pp.ParseResults):
-            return self._compose_subscript_generator(element)
+            if element.get_name() == "subfield":
+                return self._compose_subfield_generator(element)
+            elif element.get_name() == "sample loop":
+                return self._compose_sample_loop_generator(element)
 
         assert isinstance(element, str)
 
         if element.startswith("%"):
-            return self._compose_tag_generator(element)
+            return self._compose_tag_generator(element, sample_loop=sample_loop)
         else:
 
             def generate(root):
+                nonlocal element
                 variant_count = root["variant_position"].shape[0]
-                yield from itertools.repeat(element, variant_count)
+                if sample_loop:
+                    sample_count = root["sample_id"].shape[0]
+                    for _ in range(variant_count):
+                        yield itertools.repeat(element, sample_count)
+                else:
+                    yield from itertools.repeat(element, variant_count)
 
             return generate