if you running test snort.

snort -c /etc/snort/snort.conf

and get an error like this : 

ERROR: ERROR /etc/snort/rules/web-misc.rules Line 452 => unable to parse pcre regex "fn=Eye\d{4}_\d{2}.log/Rmsi"
Fatal Error, Quitting..

Solution :

you must edit file /etc/snort/rules/web-misc.rules with your favorite text editor, on Line 452 . and :

change/add line above :
pcre:"fn=Eye\d{4}_\d{2}.log/Rmsi"

with :
pcre:"/fn=Eye\d{4}_\d{2}.log/Rmsi"

Just add '/' in front line.