Make contains filter match on spaces and input boundaries, as well as word boundaries

Closes #909

DiscordChatExporter.Core/Exporting/Filtering/ContainsMessageFilter.cs

@@ -10,11 +10,19 @@ internal class ContainsMessageFilter : MessageFilter
 
     public ContainsMessageFilter(string text) => _text = text;
 
+    // Match content within word boundaries, between spaces, or as the whole input.
+    // For example, "max" shouldn't match on content "our maximum effort",
+    // but should match on content "our max effort".
+    // Also, "(max)" should match on content "our (max) effort", even though
+    // parentheses are not considered word characters.
+    // https://github.com/Tyrrrz/DiscordChatExporter/issues/909
     private bool IsMatch(string? content) =>
         !string.IsNullOrWhiteSpace(content) &&
         Regex.IsMatch(
             content,
-            "\\b" + Regex.Escape(_text) + "\\b",
+            @"(?=\b|\s|^)" +
+            Regex.Escape(_text) +
+            @"(?=\b|\s|$)",
             RegexOptions.IgnoreCase | RegexOptions.CultureInvariant
         );
 

DiscordChatExporter.Core/Exporting/Filtering/Parsing/FilterGrammar.cs

@@ -70,7 +70,7 @@ internal static class FilterGrammar
             .Named("has:<value>");
 
     // Make sure that property-based filters like 'has:link' don't prevent text like 'hello' from being parsed.
-    // https://github.com/Tyrrrz/DiscordChatExporter/issues/909
+    // https://github.com/Tyrrrz/DiscordChatExporter/issues/909#issuecomment-1227575455
     private static readonly TextParser<MessageFilter> PrimitiveFilter =
         Parse.OneOf(
             FromFilter,