[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)
[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

jordansissel/grok,jordansissel/grok