Document and fix the Index variable in loops

DocxTemplater.Test/DocxTemplater.Test.csproj

@@ -1,6 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <ImplicitUsings>enable</ImplicitUsings>
+	  <LangVersion>12.0</LangVersion>
+		<ImplicitUsings>enable</ImplicitUsings>
     <Nullable>disable</Nullable>
     <IsPackable>false</IsPackable>
     <IsTestProject>true</IsTestProject>

DocxTemplater.Test/PatternMatcherTest.cs

@@ -57,6 +57,9 @@ static IEnumerable TestPatternMatch_Cases()
             yield return new TestCaseData("{{var}:format(a,b)}").Returns(new[] { PatternType.Variable }).SetName("Multiple Arguments");
             yield return new TestCaseData("{{/}}").Returns(new[] { PatternType.ConditionEnd });
             yield return new TestCaseData("{ { / } }").Returns(new[] { PatternType.ConditionEnd });
+            yield return new TestCaseData("{?{ds.QrBills.idx == 2}}").Returns(new[] { PatternType.Condition });
+            yield return new TestCaseData("{?{ds.QrBills._Idx == 2}}").Returns(new[] { PatternType.Condition }).SetName("underscore in variable name");
+            yield return new TestCaseData("{?{ds.QrBills._Idx % 2 == 0}}").Returns(new[] { PatternType.Condition }).SetName("modulo in condition");
             yield return new TestCaseData(
                     "NumericValue is greater than 0 - {{ds.Items.InnerCollection.InnerValue}:toupper()}{{else}}" +
                     "I'm here if if this is not the case{{/}}{{/ds.Items.InnerCollection}}{{/Items}}")

DocxTemplater.Test/SpecialCollectionVariableTest.cs

@@ -0,0 +1,68 @@
+using DocumentFormat.OpenXml.Packaging;
+using DocumentFormat.OpenXml.Wordprocessing;
+using DocumentFormat.OpenXml;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace DocxTemplater.Test
+{
+    internal class SpecialCollectionVariableTest
+    {
+        [Test]
+        public void TestIndexVariableInLoop()
+        {
+            var model = new[] {"Item1", "Item2", "Item3", "Item4"};
+            var template = "Items:{{#Items}}{{Items._Idx}}{{.}} {{/Items}}";
+
+            using var memStream = new MemoryStream();
+            using var wpDocument = WordprocessingDocument.Create(memStream, WordprocessingDocumentType.Document);
+            MainDocumentPart mainPart = wpDocument.AddMainDocumentPart();
+            mainPart.Document = new Document(new Body(new Paragraph(new Run(new Text(template)))));
+            wpDocument.Save();
+            memStream.Position = 0;
+            var docTemplate = new DocxTemplate(memStream);
+            docTemplate.BindModel("Items", model);
+            var result = docTemplate.Process();
+            result.Position = 0;
+            // compare body
+            result.Position = 0;
+            var document = WordprocessingDocument.Open(result, false);
+            var body = document.MainDocumentPart.Document.Body;
+            Assert.That(body.InnerXml, Is.EqualTo("<w:p xmlns:w=\"http://schemas.openxmlformats.org/wordprocessingml/2006/main\">" +
+                                                  "<w:r>" +
+                                                  "<w:t xml:space=\"preserve\">Items:</w:t><w:t xml:space=\"preserve\">1</w:t><w:t xml:space=\"preserve\">Item1</w:t>" +
+                                                  "<w:t xml:space=\"preserve\"> </w:t><w:t xml:space=\"preserve\">2</w:t><w:t xml:space=\"preserve\">Item2</w:t>" +
+                                                  "<w:t xml:space=\"preserve\"> </w:t><w:t xml:space=\"preserve\">3</w:t><w:t xml:space=\"preserve\">Item3</w:t>" +
+                                                  "<w:t xml:space=\"preserve\"> </w:t><w:t xml:space=\"preserve\">4</w:t><w:t xml:space=\"preserve\">Item4</w:t>" +
+                                                  "<w:t xml:space=\"preserve\"> </w:t></w:r></w:p>"));
+        }
+
+        [Test]
+        public void TestConditionWithIndexVariableInLoop()
+        {
+            var model = new[] { "Item1", "Item2", "Item3", "Item4" };
+            var template = "Items:{{#Items}}{?{Items._Idx % 2 == 0}}{{.}}{{/}}{{/Items}}";
+
+            using var memStream = new MemoryStream();
+            using var wpDocument = WordprocessingDocument.Create(memStream, WordprocessingDocumentType.Document);
+            MainDocumentPart mainPart = wpDocument.AddMainDocumentPart();
+            mainPart.Document = new Document(new Body(new Paragraph(new Run(new Text(template)))));
+            wpDocument.Save();
+            memStream.Position = 0;
+            var docTemplate = new DocxTemplate(memStream);
+            docTemplate.BindModel("Items", model);
+            var result = docTemplate.Process();
+            result.Position = 0;
+            // compare body
+            result.Position = 0;
+            var document = WordprocessingDocument.Open(result, false);
+            var body = document.MainDocumentPart.Document.Body;
+            Assert.That(body.InnerXml, Is.EqualTo("<w:p xmlns:w=\"http://schemas.openxmlformats.org/wordprocessingml/2006/main\"><w:r><w:t xml:space=\"preserve\">Items:" +
+                                                  "</w:t><w:t xml:space=\"preserve\">Item2</w:t>" +
+                                                  "<w:t xml:space=\"preserve\">Item4</w:t></w:r></w:p>"));
+        }
+    }
+}

DocxTemplater/PatterMatcher.cs

@@ -25,7 +25,7 @@ internal static class PatternMatcher
                                                                     (?<separator>:\s*s\s*:) |
                                                                     (?<else>(?:else)|:) |
                                                                     (?(condMarker) # if condition marker is set, we expect a condition
-                                                                        (?<condition>[a-zA-Z0-9+\-*\/><=\s\.\!&\|]+)? #condition name (without brackets)
+                                                                        (?<condition>[a-zA-Z0-9+\-*\/><=\s\.\!&\|_%]+)? #condition expression (without brackets)
                                                                         |
                                                                         (?:
                                                                             (?<prefix>[\/\#])?(?<varname>[a-zA-Z0-9\._]+)? #variable name

README.md

@@ -77,6 +77,7 @@ The syntax is case insensitive.
 | `{{SomeDate}:F('MM/dd/yyyy')}`      | Date variable with formatting - short syntax                   |
 | `{{SomeBytes}:img()}`               | Image Formatter for image data                                 |
 | `{{SomeHtmlString}:html()}`         | Inserts HTML string into the word document                     |
+| `{{#Items}}{?{Items._Idx % 2 == 0}}{{.}}{{/}}{{/Items}}`         | Renders every second item in a list                 |
 ---
 ### Collections
 
@@ -113,6 +114,11 @@ Will render a table row for each item in the collection:
 |-------|-----------|
 | John  | Developer |
 | Alice | CEO       |
+
+#### Accessing the Index of the Current Item
+
+To access the index of the current item, use the special variable `Items._Idx` In this example, the collection is called "Items".
+
 ---
 ### Separator