diff --git a/packages/tilia-core/src/tilia_core/timemap/pattern.py b/packages/tilia-core/src/tilia_core/timemap/pattern.py
new file mode 100644
index 000000000..236c76e56
--- /dev/null
+++ b/packages/tilia-core/src/tilia_core/timemap/pattern.py
@@ -0,0 +1,197 @@
+"""
+Parsing for the beat pattern syntax.
+
+A pattern is a whitespace-separated list of items. An item is either a bar
+length or a repeated group of items::
+
+    items := item (whitespace item)*
+    item  := INT | INT? "[" items "]"
+
+So ``4 4 3`` is three bars, ``10[4] 3 15[4]`` is ten bars of 4, one of 3 and
+fifteen of 4, and ``2[4 3[3]]`` expands to ``4 3 3 3 4 3 3 3``. A group with no
+count (``[4 3]``) is played once.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+from enum import Enum, auto
+
+# Guards against inputs like "999999[999999[4]]" expanding into a list that
+# would exhaust memory. Far above the bar count of any real piece.
+MAX_BARS = 100_000
+# Groups are parsed recursively, so nesting is capped well below Python's
+# recursion limit. Far deeper than any pattern worth writing.
+MAX_DEPTH = 32
+# Significant digits in a number. Any more and the number is far above
+# MAX_BARS, and int() refuses strings past a few thousand digits.
+MAX_DIGITS = 9
+# Only ASCII digits: str.isdigit() also accepts characters such as "²" that
+# int() rejects.
+_DIGITS = frozenset("0123456789")
+
+
+class ParseStatus(Enum):
+    COMPLETE = auto()
+    # A valid start that more typing could complete, e.g. "10[" or "".
+    INCOMPLETE = auto()
+    INVALID = auto()
+
+
+@dataclass(frozen=True)
+class ParseResult:
+    status: ParseStatus
+    bars: list[int] = field(default_factory=list)
+    error: str = ""
+    position: int | None = None
+
+    @property
+    def is_complete(self) -> bool:
+        return self.status == ParseStatus.COMPLETE
+
+
+@dataclass(frozen=True)
+class BarRun:
+    """Consecutive bars with the same length. `start` is a 0-based bar index."""
+
+    start: int
+    count: int
+    beats: int
+
+
+class _Incomplete(Exception):
+    def __init__(self, message: str, position: int) -> None:
+        super().__init__(message)
+        self.message = message
+        self.position = position
+
+
+class _Invalid(Exception):
+    def __init__(self, message: str, position: int) -> None:
+        super().__init__(message)
+        self.message = message
+        self.position = position
+
+
+class _Parser:
+    def __init__(self, text: str) -> None:
+        self.text = text
+        self.pos = 0
+        self.depth = 0
+
+    def at_end(self) -> bool:
+        return self.pos >= len(self.text)
+
+    def peek(self) -> str:
+        return self.text[self.pos]
+
+    def at_digit(self) -> bool:
+        return not self.at_end() and self.peek() in _DIGITS
+
+    def skip_whitespace(self) -> None:
+        while not self.at_end() and self.peek().isspace():
+            self.pos += 1
+
+    def parse_int(self) -> int:
+        start = self.pos
+        while self.at_digit():
+            self.pos += 1
+        digits = self.text[start : self.pos].lstrip("0")
+        if len(digits) > MAX_DIGITS:
+            raise _Invalid(f"Numbers can have at most {MAX_DIGITS} digits.", start)
+        return int(digits or "0")
+
+    def parse_items(self, open_position: int | None = None) -> list[int]:
+        """
+        Parses items up to the end of the text, or up to the "]" closing the
+        group opened at `open_position`. Leaves that "]" unconsumed.
+        """
+        in_group = open_position is not None
+        bars: list[int] = []
+        while True:
+            self.skip_whitespace()
+            if self.at_end():
+                if in_group:
+                    raise _Incomplete("Unclosed '['.", open_position)
+                break
+            if self.peek() == "]":
+                if in_group:
+                    break
+                raise _Invalid("Unmatched ']'.", self.pos)
+            item_start = self.pos
+            bars.extend(self.parse_item())
+            if len(bars) > MAX_BARS:
+                raise _Invalid(
+                    f"Pattern expands to more than {MAX_BARS} bars.", item_start
+                )
+
+        if not bars:
+            if in_group:
+                raise _Invalid("Empty group '[]'.", open_position)
+            raise _Incomplete("Enter at least one bar length.", 0)
+
+        return bars
+
+    def parse_item(self) -> list[int]:
+        start = self.pos
+        count = self.parse_int() if self.at_digit() else None
+        self.skip_whitespace()
+
+        if not self.at_end() and self.peek() == "[":
+            open_position = self.pos
+            if self.depth >= MAX_DEPTH:
+                raise _Invalid(
+                    f"Groups can be nested at most {MAX_DEPTH} deep.", open_position
+                )
+            self.pos += 1
+            self.depth += 1
+            group = self.parse_items(open_position)
+            self.depth -= 1
+            self.pos += 1  # the closing "]"
+            repeats = 1 if count is None else count
+            if repeats < 1:
+                raise _Invalid("Repeat count must be at least 1.", start)
+            if len(group) * repeats > MAX_BARS:
+                raise _Invalid(f"Pattern expands to more than {MAX_BARS} bars.", start)
+            return group * repeats
+
+        if count is None:
+            raise _Invalid(f"Unexpected '{self.peek()}'.", self.pos)
+        if count < 1:
+            raise _Invalid("Bar lengths must be at least 1.", start)
+        return [count]
+
+
+def parse(text: str) -> ParseResult:
+    """Parses a beat pattern into the list of bar lengths it expands to."""
+    try:
+        bars = _Parser(text).parse_items()
+    except _Incomplete as e:
+        return ParseResult(ParseStatus.INCOMPLETE, error=e.message, position=e.position)
+    except _Invalid as e:
+        return ParseResult(ParseStatus.INVALID, error=e.message, position=e.position)
+
+    return ParseResult(ParseStatus.COMPLETE, bars)
+
+
+def group_runs(bars: list[int]) -> list[BarRun]:
+    """Groups consecutive bars of equal length, e.g. for previewing a pattern."""
+    runs: list[BarRun] = []
+    for index, beats in enumerate(bars):
+        if runs and runs[-1].beats == beats:
+            last = runs[-1]
+            runs[-1] = BarRun(last.start, last.count + 1, beats)
+        else:
+            runs.append(BarRun(index, 1, beats))
+    return runs
+
+
+def format_bars(bars: list[int]) -> str:
+    """
+    Writes bar lengths as a pattern, collapsing runs of equal bars:
+    ``[4, 4, 4, 3]`` becomes ``3[4] 3``. Nested groups are not recovered.
+    """
+    return " ".join(
+        f"{run.count}[{run.beats}]" if run.count > 1 else str(run.beats)
+        for run in group_runs(bars)
+    )
diff --git a/packages/tilia-core/tests/timemap/test_timemap_pattern.py b/packages/tilia-core/tests/timemap/test_timemap_pattern.py
new file mode 100644
index 000000000..90d052b86
--- /dev/null
+++ b/packages/tilia-core/tests/timemap/test_timemap_pattern.py
@@ -0,0 +1,171 @@
+import pytest
+
+from tilia_core.timemap.pattern import (
+    MAX_BARS,
+    MAX_DEPTH,
+    MAX_DIGITS,
+    BarRun,
+    ParseStatus,
+    format_bars,
+    group_runs,
+    parse,
+)
+
+
+class TestComplete:
+    @pytest.mark.parametrize(
+        "text, bars",
+        [
+            ("4", [4]),
+            ("4 4 3", [4, 4, 3]),
+            ("  4\t4\n3  ", [4, 4, 3]),
+            ("10[4] 3 15[4]", [4] * 10 + [3] + [4] * 15),
+            ("2[4 3[3]]", [4, 3, 3, 3, 4, 3, 3, 3]),
+            ("2[[4] 3[3]]", [4, 3, 3, 3, 4, 3, 3, 3]),
+            ("[4 3]", [4, 3]),
+            ("1[4]", [4]),
+            ("3 [2]", [2, 2, 2]),
+            ("2[3]4", [3, 3, 4]),
+            ("12", [12]),
+            ("007", [7]),
+        ],
+    )
+    def test_expands(self, text, bars):
+        result = parse(text)
+        assert result.status == ParseStatus.COMPLETE
+        assert result.is_complete
+        assert result.bars == bars
+        assert result.error == ""
+
+    def test_max_bars_is_allowed(self):
+        assert parse(f"{MAX_BARS}[1]").bars == [1] * MAX_BARS
+
+
+class TestIncomplete:
+    @pytest.mark.parametrize(
+        "text, position",
+        [
+            ("", 0),
+            ("   ", 0),
+            ("10[", 2),
+            ("2[4 3", 1),
+            ("2[4 3[3]", 1),
+            ("2[4 3[3", 5),
+        ],
+    )
+    def test_incomplete(self, text, position):
+        result = parse(text)
+        assert result.status == ParseStatus.INCOMPLETE
+        assert not result.is_complete
+        assert result.bars == []
+        assert result.error
+        assert result.position == position
+
+
+class TestInvalid:
+    @pytest.mark.parametrize(
+        "text, position, message",
+        [
+            ("4 ]", 2, "Unmatched ']'."),
+            ("2[4]]", 4, "Unmatched ']'."),
+            ("4 a", 2, "Unexpected 'a'."),
+            ("4, 4", 1, "Unexpected ','."),
+            ("-4", 0, "Unexpected '-'."),
+            ("0", 0, "Bar lengths must be at least 1."),
+            ("4 0", 2, "Bar lengths must be at least 1."),
+            ("0[4]", 0, "Repeat count must be at least 1."),
+            ("2[]", 1, "Empty group '[]'."),
+            ("2[ ]", 1, "Empty group '[]'."),
+        ],
+    )
+    def test_invalid(self, text, position, message):
+        result = parse(text)
+        assert result.status == ParseStatus.INVALID
+        assert result.bars == []
+        assert result.error == message
+        assert result.position == position
+
+    @pytest.mark.parametrize(
+        "text",
+        [f"{MAX_BARS + 1}[1]", "999999999[999999999[4]]", f"{MAX_BARS}[1] 1"],
+    )
+    def test_too_many_bars(self, text):
+        result = parse(text)
+        assert result.status == ParseStatus.INVALID
+        assert "more than" in result.error
+
+    @pytest.mark.parametrize("text", ["²", "4 ²", "①[4]", "٣"])
+    def test_non_ascii_digits(self, text):
+        result = parse(text)
+        assert result.status == ParseStatus.INVALID
+        assert result.error.startswith("Unexpected")
+
+    @pytest.mark.parametrize("text", ["9" * 5000, f"{'9' * 10}[4]"])
+    def test_too_many_digits(self, text):
+        result = parse(text)
+        assert result.status == ParseStatus.INVALID
+        assert result.error == f"Numbers can have at most {MAX_DIGITS} digits."
+        assert result.position == 0
+
+    def test_leading_zeros_do_not_count_as_digits(self):
+        assert parse("0" * 20 + "4").bars == [4]
+
+    def test_max_depth_is_allowed(self):
+        text = "[" * MAX_DEPTH + "4" + "]" * MAX_DEPTH
+        assert parse(text).bars == [4]
+
+    @pytest.mark.parametrize("closed", [True, False])
+    def test_too_deep(self, closed):
+        depth = MAX_DEPTH + 1
+        text = "[" * depth + "4" + ("]" * depth if closed else "")
+        result = parse(text)
+        assert result.status == ParseStatus.INVALID
+        assert result.error == f"Groups can be nested at most {MAX_DEPTH} deep."
+        assert result.position == MAX_DEPTH
+
+    def test_very_deep_nesting_does_not_recurse(self):
+        result = parse("[" * 5000 + "4" + "]" * 5000)
+        assert result.status == ParseStatus.INVALID
+
+
+class TestGroupRuns:
+    def test_empty(self):
+        assert group_runs([]) == []
+
+    def test_groups_consecutive_equal_bars(self):
+        assert group_runs([4] * 10 + [3] + [4] * 15) == [
+            BarRun(0, 10, 4),
+            BarRun(10, 1, 3),
+            BarRun(11, 15, 4),
+        ]
+
+    def test_does_not_merge_non_consecutive_bars(self):
+        assert group_runs([4, 3, 4]) == [
+            BarRun(0, 1, 4),
+            BarRun(1, 1, 3),
+            BarRun(2, 1, 4),
+        ]
+
+
+class TestFormatBars:
+    @pytest.mark.parametrize(
+        "bars, text",
+        [
+            ([4], "4"),
+            ([4, 4, 4, 3], "3[4] 3"),
+            ([4] * 10 + [3] + [4] * 15, "10[4] 3 15[4]"),
+            ([4, 3, 4], "4 3 4"),
+        ],
+    )
+    def test_collapses_runs_of_equal_bars(self, bars, text):
+        assert format_bars(bars) == text
+
+    def test_does_not_recover_nested_groups(self):
+        assert format_bars(parse("2[4 3]").bars) == "4 3 4 3"
+
+    @pytest.mark.parametrize(
+        "text", ["4 4 3", "10[4] 3 15[4]", "2[4 3[3]]", f"{MAX_BARS}[1]"]
+    )
+    def test_parses_back_to_the_same_bars(self, text):
+        bars = parse(text).bars
+        assert parse(format_bars(bars)).bars == bars