diff --git a/DevProxy.Abstractions/Models/MockStdioResponse.cs b/DevProxy.Abstractions/Models/MockStdioResponse.cs
index 4a39a24e..47e01132 100644
--- a/DevProxy.Abstractions/Models/MockStdioResponse.cs
+++ b/DevProxy.Abstractions/Models/MockStdioResponse.cs
@@ -34,11 +34,21 @@ public object Clone()
 public class MockStdioRequest
 {
     /// <summary>
-    /// A fragment of the stdin body to match.
-    /// If null or empty, the mock matches any stdin (or is applied immediately on startup).
+    /// A fragment of the stdin body to match (case-insensitive contains).
+    /// Prefer using either <see cref="BodyFragment"/> or <see cref="BodyRegex"/>, not both.
+    /// If both are specified, <see cref="BodyRegex"/> takes precedence and <see cref="BodyFragment"/> is ignored.
+    /// If neither is specified, the mock matches any stdin (or is applied immediately on startup).
     /// </summary>
     public string? BodyFragment { get; set; }
 
+    /// <summary>
+    /// A regular expression pattern to match against the stdin body (case-insensitive).
+    /// Prefer using either <see cref="BodyRegex"/> or <see cref="BodyFragment"/>, not both.
+    /// If both are specified, <see cref="BodyRegex"/> takes precedence and <see cref="BodyFragment"/> is ignored.
+    /// If neither is specified, the mock matches any stdin (or is applied immediately on startup).
+    /// </summary>
+    public string? BodyRegex { get; set; }
+
     /// <summary>
     /// The Nth occurrence to match. If null, matches every occurrence.
     /// </summary>
diff --git a/DevProxy.Plugins/Mocking/MockStdioResponsePlugin.cs b/DevProxy.Plugins/Mocking/MockStdioResponsePlugin.cs
index 09a38322..885b2207 100644
--- a/DevProxy.Plugins/Mocking/MockStdioResponsePlugin.cs
+++ b/DevProxy.Plugins/Mocking/MockStdioResponsePlugin.cs
@@ -202,7 +202,8 @@ private void ApplyStartupMocks(StdioRequestArgs e)
 
         // Find mocks without stdin patterns (startup mocks)
         var startupMocks = Configuration.Mocks
-            .Where(m => string.IsNullOrEmpty(m.Request?.BodyFragment))
+            .Where(m => string.IsNullOrEmpty(m.Request?.BodyFragment) &&
+                        string.IsNullOrEmpty(m.Request?.BodyRegex))
             .ToList();
 
         foreach (var mock in startupMocks)
@@ -249,16 +250,42 @@ Configuration.Mocks is null ||
                 return false;
             }
 
-            if (string.IsNullOrEmpty(mockResponse.Request.BodyFragment))
+            var hasBodyFragment = !string.IsNullOrEmpty(mockResponse.Request.BodyFragment);
+            var hasBodyRegex = !string.IsNullOrEmpty(mockResponse.Request.BodyRegex);
+
+            if (!hasBodyFragment && !hasBodyRegex)
             {
-                // No body fragment means startup mock, handled separately
+                // No body fragment or regex means startup mock, handled separately
                 return false;
             }
 
-            // Check if stdin contains the body fragment
-            if (!stdinBody.Contains(mockResponse.Request.BodyFragment, StringComparison.OrdinalIgnoreCase))
+            // Use bodyRegex if specified, otherwise use bodyFragment (either/or)
+            if (hasBodyRegex)
             {
-                return false;
+                try
+                {
+                    if (!Regex.IsMatch(stdinBody, mockResponse.Request.BodyRegex!, RegexOptions.IgnoreCase, TimeSpan.FromSeconds(5)))
+                    {
+                        return false;
+                    }
+                }
+                catch (RegexMatchTimeoutException ex)
+                {
+                    Logger.LogError(ex, "Regex match timed out for body regex pattern '{Pattern}'. Treating as non-matching mock.", mockResponse.Request.BodyRegex);
+                    return false;
+                }
+                catch (ArgumentException ex)
+                {
+                    Logger.LogError(ex, "Invalid body regex pattern '{Pattern}'. Treating as non-matching mock.", mockResponse.Request.BodyRegex);
+                    return false;
+                }
+            }
+            else if (hasBodyFragment)
+            {
+                if (!stdinBody.Contains(mockResponse.Request.BodyFragment!, StringComparison.OrdinalIgnoreCase))
+                {
+                    return false;
+                }
             }
 
             // Check Nth condition
@@ -267,7 +294,7 @@ Configuration.Mocks is null ||
 
         if (mockResponse?.Request is not null)
         {
-            var mockKey = mockResponse.Request.BodyFragment ?? "default";
+            var mockKey = GetMockKey(mockResponse);
             _ = _appliedMocks.AddOrUpdate(mockKey, 1, (_, value) => ++value);
         }
 
@@ -282,13 +309,20 @@ private bool IsNthRequest(MockStdioResponse mockResponse)
             return true;
         }
 
-        var mockKey = mockResponse.Request.BodyFragment ?? "default";
+        var mockKey = GetMockKey(mockResponse);
         _ = _appliedMocks.TryGetValue(mockKey, out var nth);
         nth++;
 
         return mockResponse.Request.Nth == nth;
     }
 
+    private static string GetMockKey(MockStdioResponse mockResponse)
+    {
+        return mockResponse.Request?.BodyRegex
+            ?? mockResponse.Request?.BodyFragment
+            ?? "default";
+    }
+
     private void ProcessMockResponse(StdioRequestArgs e, MockStdioResponse matchingResponse)
     {
         // Replace placeholders in the response with values from stdin
diff --git a/schemas/v2.2.0/mockstdioresponseplugin.mocksfile.schema.json b/schemas/v2.2.0/mockstdioresponseplugin.mocksfile.schema.json
index 23582644..3579f028 100644
--- a/schemas/v2.2.0/mockstdioresponseplugin.mocksfile.schema.json
+++ b/schemas/v2.2.0/mockstdioresponseplugin.mocksfile.schema.json
@@ -20,7 +20,11 @@
             "properties": {
               "bodyFragment": {
                 "type": "string",
-                "description": "(Optional) A fragment of the stdin body to match. If null or empty, the mock matches any stdin or is applied immediately on startup."
+                "description": "(Optional) A fragment of the stdin body to match (case-insensitive contains). You can specify bodyFragment, bodyRegex, or both; if both are specified, bodyRegex takes precedence. If neither is specified, the mock matches any stdin or is applied immediately on startup."
+              },
+              "bodyRegex": {
+                "type": "string",
+                "description": "(Optional) A regular expression pattern to match against the stdin body (case-insensitive). You can specify bodyRegex, bodyFragment, or both; if both are specified, bodyRegex takes precedence. If neither is specified, the mock matches any stdin or is applied immediately on startup."
               },
               "nth": {
                 "type": "integer",