Markers now allow properties beginning with digits

Previously, we required property names to begin with a letter or an
underscore,but this requirement wasn't really necessary, because there's
no ambiguity avoided here (additionally, it prevented [select] from
working with numbers)

CHANGELOG.md

@@ -6,6 +6,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## [Unreleased]
 
+### Added
+
+- Markup attributes may now begin with a digit, letter or underscore. Previously, they were required to begin with a letter or an underscore. This allows the `select` marker to work with numbers: `[select value=1 1=one 2=two 3=three /]`
+
 ## [2.2.0] 2022-04-08
 
 ### Added

YarnSpinner.Tests/MarkupTests.cs

@@ -295,5 +295,21 @@ public void TestMarkupEscaping() {
             Assert.Equal(0, markup.Attributes[0].Position);
             Assert.Equal(18, markup.Attributes[0].Length);
         }
+
+        [Fact]
+        public void TestNumericProperties() {
+            var line = @"[select value=1 1=one 2=two 3=three /]";
+            var markup = dialogue.ParseMarkup(line);
+
+            Assert.Single(markup.Attributes);
+            Assert.Equal("select", markup.Attributes[0].Name);
+            Assert.Equal(4, markup.Attributes[0].Properties.Count);
+            Assert.Equal(1, markup.Attributes[0].Properties["value"].IntegerValue);
+            Assert.Equal("one", markup.Attributes[0].Properties["1"].StringValue);
+            Assert.Equal("two", markup.Attributes[0].Properties["2"].StringValue);
+            Assert.Equal("three", markup.Attributes[0].Properties["3"].StringValue);
+
+            Assert.Equal("one", markup.Text);
+        }
     }
 }

YarnSpinner/YarnSpinner.Markup/LineParser.cs

@@ -787,7 +787,7 @@ private string ParseID()
             this.ConsumeWhitespace();
             var idStringBuilder = new StringBuilder();
 
-            // Read the first character, which must be a letter
+            // Read the first character, which must be a letter, number, or underscore
             int tempNext = this.stringReader.Read();
             this.sourcePosition += 1;
             this.AssertNotEndOfInput(tempNext);
@@ -805,7 +805,7 @@ private string ParseID()
                 idStringBuilder.Append(nextChar);
                 idStringBuilder.Append(nextNextChar);
             }
-            else if (char.IsLetter(nextChar) || nextChar == '_')
+            else if (char.IsLetterOrDigit(nextChar) || nextChar == '_')
             {
                 idStringBuilder.Append((char)tempNext);
             }