[fix] grok is not working anymore with PCRE >= 8.34 because the latter
does not allow anymore a group name to start with digits (PCRE Changelog
version 8.34, point 36)

this commit fix the issue by prepending a '_' at the start of group
name. It remains compatible with older versions of PCRE

https://github.com/jordansissel/grok/pull/23
https://github.com/jordansissel/grok/commit/ecb6f35854795242c3fd7b33a499ed8896af8de4
--- grokre.c.orig
+++ 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);