diff --git a/src/StreamCore.cc b/src/StreamCore.cc
index d598a2e..21ca523 100644
--- a/src/StreamCore.cc
+++ b/src/StreamCore.cc
@@ -1166,14 +1166,21 @@ matchInput()
                     }
                     if (consumed < 0)
                     {
-                        if (!(flags & AsyncMode) && onMismatch[0] != in_cmd)
+                        if (fmt.flags & default_flag)
                         {
-                            error("%s: Input \"%s%s\" does not match format %%%s\n",
-                                name(), inputLine.expand(consumedInput, 20)(),
-                                inputLine.length()-consumedInput > 20 ? "..." : "",
-                                formatstring);
+                            consumed = 0;
+                        }
+                        else
+                        {
+                            if (!(flags & AsyncMode) && onMismatch[0] != in_cmd)
+                            {
+                                error("%s: Input \"%s%s\" does not match format %%%s\n",
+                                    name(), inputLine.expand(consumedInput, 20)(),
+                                    inputLine.length()-consumedInput > 20 ? "..." : "",
+                                    formatstring);
+                            }
+                            return false;
                         }
-                        return false;
                     }
                     consumedInput += consumed;
                     break;
@@ -1323,8 +1330,16 @@ scanValue(const StreamFormat& fmt, long& value)
         scanLong(fmt, inputLine(consumedInput), value);
     debug("StreamCore::scanValue(%s, format=%%%c, long) input=\"%s\"\n",
         name(), fmt.conv, inputLine.expand(consumedInput)());
-    if (consumed < 0 ||
-        consumed > inputLine.length()-consumedInput) return -1;
+    if (consumed < 0)
+    {
+        if (fmt.flags & default_flag)
+        {
+            value = 0;
+            consumed = 0;
+        }
+        else return -1;
+    }
+    if (consumed > inputLine.length()-consumedInput) return -1;
     debug("StreamCore::scanValue(%s) scanned %li\n",
         name(), value);
     flags |= GotValue;
@@ -1346,8 +1361,16 @@ scanValue(const StreamFormat& fmt, double& value)
         scanDouble(fmt, inputLine(consumedInput), value);
     debug("StreamCore::scanValue(%s, format=%%%c, double) input=\"%s\"\n",
         name(), fmt.conv, inputLine.expand(consumedInput)());
-    if (consumed < 0 ||
-        consumed > inputLine.length()-consumedInput) return -1;
+    if (consumed < 0)
+    {
+        if (fmt.flags & default_flag)
+        {
+            value = 0.0;
+            consumed = 0;
+        }
+        else return -1;
+    }
+    if (consumed > inputLine.length()-consumedInput) return -1;
     debug("StreamCore::scanValue(%s) scanned %#g\n",
         name(), value);
     flags |= GotValue;
@@ -1370,8 +1393,16 @@ scanValue(const StreamFormat& fmt, char* value, long maxlen)
         scanString(fmt, inputLine(consumedInput), value, maxlen);
     debug("StreamCore::scanValue(%s, format=%%%c, char*, maxlen=%ld) input=\"%s\"\n",
         name(), fmt.conv, maxlen, inputLine.expand(consumedInput)());
-    if (consumed < 0 ||
-        consumed > inputLine.length()-consumedInput) return -1;
+    if (consumed < 0)
+    {
+        if (fmt.flags & default_flag)
+        {
+            value[0] = 0;
+            consumed = 0;
+        }
+        else return -1;
+    }
+    if (consumed > inputLine.length()-consumedInput) return -1;
 #ifndef NO_TEMPORARY
     debug("StreamCore::scanValue(%s) scanned \"%s\"\n",
         name(), StreamBuffer(value, maxlen).expand()());
diff --git a/src/StreamFormat.h b/src/StreamFormat.h
index 7e049e8..cac9ff1 100644
--- a/src/StreamFormat.h
+++ b/src/StreamFormat.h
@@ -23,12 +23,13 @@
 #define StreamFormat_h
 
 typedef enum {
-    left_flag  = 0x01,
-    sign_flag  = 0x02,
-    space_flag = 0x04,
-    alt_flag   = 0x08,
-    zero_flag  = 0x10,
-    skip_flag  = 0x20
+    left_flag    = 0x01,
+    sign_flag    = 0x02,
+    space_flag   = 0x04,
+    alt_flag     = 0x08,
+    zero_flag    = 0x10,
+    skip_flag    = 0x20,
+    default_flag = 0x40
 } StreamFormatFlag;
 
 typedef enum {
diff --git a/src/StreamFormatConverter.cc b/src/StreamFormatConverter.cc
index 30a8686..d71c7fc 100644
--- a/src/StreamFormatConverter.cc
+++ b/src/StreamFormatConverter.cc
@@ -112,7 +112,7 @@ static void copyFormatString(StreamBuffer& info, const char* source)
     const char* p = source - 1;
     while (*p != '%' && *p != ')') p--;
     info.append('%');
-    while (++p != source-1) info.append(*p);
+    while (++p != source-1) if (*p != '?') info.append(*p);
 }
 
 // Standard Long Converter for 'diouxX'
@@ -278,7 +278,7 @@ scanString(const StreamFormat& fmt, const char* input,
     if (*input == '\0')
     {
         // match empty string
-        value[0] = '\0';
+        if (value) value[0] = '\0';
         return 0;
     }
     if (fmt.flags & skip_flag)
diff --git a/src/StreamProtocol.cc b/src/StreamProtocol.cc
index 7f5d2f8..446a51e 100644
--- a/src/StreamProtocol.cc
+++ b/src/StreamProtocol.cc
@@ -1504,6 +1504,16 @@ compileFormat(StreamBuffer& buffer, const char*& formatstr,
                 }
                 streamFormat.flags |= skip_flag;
                 break;
+            case '?':
+                if (formatType != ScanFormat)
+                {
+                    errorMsg(line,
+                        "Use of default modifier '?' "
+                        "only allowed in input formats\n");
+                    return false;
+                }
+                streamFormat.flags |= default_flag;
+                break;
             default:
                 loop = false;
         }