add grouping syntax

package.json

@@ -1,6 +1,6 @@
 {
   "name": "mehrzahl",
-  "version": "0.0.1-alpha2",
+  "version": "0.0.1-alpha3",
   "description": "Tagged template literals for singular/plural formatting",
   "source": "src/index.ts",
   "main": "lib/index.js",

src/__tests__/mehrzahl.spec.ts

@@ -23,10 +23,12 @@ describe("Mehrzahl function", () => {
     const formatSingular = mz(1)
     const formatPlural = mz(5)
 
-    expect(formatSingular`number is ${[null, 'plural']}`).toEqual("number is ")
-    expect(formatSingular`number is ${['singular']}`).toEqual("number is singular")
-    expect(formatPlural`number is ${[, 'plural']}`).toEqual("number is plural")
-    expect(formatPlural`number is ${['singular']}`).toEqual("number is ")
+    expect(formatSingular`number is ${[null, "plural"]}`).toEqual("number is ")
+    expect(formatSingular`number is ${["singular"]}`).toEqual(
+      "number is singular"
+    )
+    expect(formatPlural`number is ${[, "plural"]}`).toEqual("number is plural")
+    expect(formatPlural`number is ${["singular"]}`).toEqual("number is ")
   })
 
   it("interpolates singular/plural tuples and function expressions", () => {
@@ -39,4 +41,40 @@ describe("Mehrzahl function", () => {
     expect(resultSingular).toEqual("number is singular singularFunction")
     expect(resultPlural).toEqual("number is plural pluralFunction")
   })
+
+  it("transforms {<singular>|<plural>} syntax", () => {
+    const resultSingular = mz(1)`number is {singular|plural}`
+
+    const resultPlural = mz(5)`number is {singular|plural}`
+
+    expect(resultSingular).toEqual("number is singular")
+    expect(resultPlural).toEqual("number is plural")
+  })
+
+  it("transforms {<singular>|<plural>} syntax with empty strings", () => {
+    const resultSingular = mz(1)`number is {singular|}`
+
+    const resultPlural = mz(5)`number is {|plural}`
+
+    expect(resultSingular).toEqual("number is singular")
+    expect(resultPlural).toEqual("number is plural")
+  })
+
+  it("transforms nested groups", () => {
+    const resultSingular = mz(1)`number is {singular ($value)|}`
+
+    const resultPlural = mz(5)`number is {|plural ($value)}`
+
+    expect(resultSingular).toEqual("number is singular (1)")
+    expect(resultPlural).toEqual("number is plural (5)")
+  })
+
+  it("replaces $value", () => {
+    const resultSingular = mz(1)`number is {singular ($value)|}`
+
+    const resultPlural = mz(5)`number is {|plural ($value)}`
+
+    expect(resultSingular).toEqual("number is singular (1)")
+    expect(resultPlural).toEqual("number is plural (5)")
+  })
 })

src/mehrzahl.ts

@@ -4,7 +4,8 @@ export type MehrzahlTaggedFormatter = (
 ) => string
 
 export type MehrzahlFormatterFactory = (
-  amount: number
+  amount: number,
+  delimiter?: string
 ) => MehrzahlTaggedFormatter
 
 export type InterpolationFunction = (plural: boolean, amount: number) => string
@@ -20,11 +21,28 @@ export type InterpolatableValue =
   | number
   | InterpolationFunction
 
+const DEFAULT_DELIMITER = "|"
+const GROUPING_REGEX = /\{(.*?)\}/g
+
 export const mz: MehrzahlFormatterFactory =
-  (amount) =>
+  (amount, delimiter = DEFAULT_DELIMITER) =>
   (strings, ...valuesToInterpolate) => {
     const isPlural = amount > 1
 
+    const formatGroupSyntax = (value: string) =>
+      value.replace(GROUPING_REGEX, (_, firstSubmatch) => {
+        if (typeof firstSubmatch === "string") {
+          const [singular, plural] = firstSubmatch.split(delimiter)
+          return isPlural ? plural : singular
+        }
+
+        return firstSubmatch
+      })
+
+    if (strings.length === 1) {
+      return formatGroupSyntax(strings[0]).replace("$value", amount.toString())
+    }
+
     const interpolatedValues = valuesToInterpolate.map((value) => {
       if (typeof value === "function") {
         return value(isPlural, amount)
@@ -34,13 +52,14 @@ export const mz: MehrzahlFormatterFactory =
         return value[isPlural ? 1 : 0]?.toString() ?? ""
       }
 
-      // ToDo: Interpolate {|} syntax
       return value.toString()
     })
 
-    return strings.reduce(
-      (result, part, index) =>
-        result + part + (interpolatedValues[index] ?? ""),
-      ""
-    )
+    return formatGroupSyntax(
+      strings.reduce(
+        (result, part, index) =>
+          result + part + (interpolatedValues[index] ?? ""),
+        ""
+      )
+    ).replace("$value", amount.toString())
   }