Try a Nearley grammar

packages/loot-core/src/server/budget/goal-template.ne

@@ -0,0 +1,67 @@
+@builtin "whitespace.ne"
+
+Expression -> (SimpleExpression | SpendByExpression | PercentageExpression | WeeksExpression)
+  {% ([[expr]]) => expr %}
+
+@{%
+const str = arr => arr.flat(Infinity).map(s => s || "").join("")
+const num = arr => +str(arr)
+%}
+
+SimpleExpression ->
+    Limit {% ([limit]) => ({ type: 'simple', limit }) %}
+  | Amount (__ Limit):? {% ([monthly, limit]) => ({type: 'simple', monthly, limit: limit?.[1] }) %}
+
+SpendByExpression ->
+  Amount __ by __ Month (__ spend_from __ Month):? (__ repeat_every __ RepeatInterval):?
+  {% ([amount, , , , month, from, repeat]) => ({
+    type: from?.[3] ? "spend" : "by",
+    amount,
+    month,
+    from: from?.[3],
+    ...(repeat?.[3] ?? {})
+  }) %}
+
+RepeatInterval ->
+    "month"i             {% () => ({ annual: false }) %}
+  | Digit:+ __ "months"i {% ([months]) => ({ annual: false, repeat: num(months) }) %}
+  | "year"i              {% () => ({ annual: true }) %}
+  | Digit:+ __ "years"i  {% ([years]) => ({ annual: true, repeat: num(years) }) %}
+
+WeeksExpression ->
+  Amount __ repeat_every __ NumberOfWeeks __ starting __ Day (__ Limit):?
+  {% ([amount, , , , weeks, , , , starting, limit]) => ({ type: 'week', amount, weeks, starting, limit: limit?.[1] }) %}
+
+NumberOfWeeks ->
+    "week"i             {% () => null %}
+  | Digit:+ __ "weeks"i {% ([weeks]) => num(weeks) %}
+
+PercentageExpression ->
+  Percent __ of __ .:*
+  {% ([percent, , , , category]) => ({ type: "percentage", percent, category: str(category) }) %}
+
+# Numbers
+
+Limit -> up_to __ Amount {% ([, , limit]) => limit %}
+
+Amount -> ("$" _):? Digit:+ ("." Digit Digit):?
+  {% (d) => +str(d.slice(1)) %}
+
+Percent -> Digit:+ _ "%"
+  {% (d) => +str(d[0]) %}
+
+# Words
+by -> "by"i
+of -> "of"i
+repeat_every -> "repeat"i __ "every"i
+starting -> "starting"i
+up_to -> "up"i __ "to"i
+spend_from -> "spend"i __ "from"i
+
+# Dates
+Year -> Digit Digit Digit Digit {% str %}
+Month -> Year "-" Digit Digit  {% str %}
+Day -> Month "-" Digit Digit {% str %}
+
+# Basic Types
+Digit -> [0-9] {% str %}