https://github.com/jordansissel/grok/commit/44ce9372a29319c01d4ac7f24287fb36315e0e34

--- a/grokre.c
+++ b/grokre.c
@@ -12,8 +12,8 @@
 
 /* global, static variables */
 
-#define CAPTURE_ID_LEN 4
-#define CAPTURE_FORMAT "%04x"
+#define CAPTURE_ID_LEN 5
+#define CAPTURE_FORMAT "_%04x"
 
 /* internal functions */
 static char *grok_pattern_expand(grok_t *grok); //, int offset, int length);