diff --git a/samples/bip.conf b/samples/bip.conf
index 68e6976..c89dae3 100644
--- a/samples/bip.conf
+++ b/samples/bip.conf
@@ -37,7 +37,7 @@ log_level = 3;
 # %m -> 2 digit month
 # %d -> 2 digit day
 # %c -> destination (#chan, privates, ...)
-#log_format = "%n/%Y-%m/%c.%d.log";
+#log_format = "%u/%n/%Y-%m/%c.%d.log";
 
 # Sets the frequency (in seconds) of log syncing (real write to kernel)
 #log_sync_interval = 5;
diff --git a/src/bip.c b/src/bip.c
index ba1192c..89f1e08 100644
--- a/src/bip.c
+++ b/src/bip.c
@@ -17,6 +17,7 @@
 #include <unistd.h>
 #include <signal.h>
 #include <stdarg.h>
+#include <string.h>
 #include <sys/time.h>
 #include <sys/resource.h>
 #include "irc.h"
@@ -704,6 +705,9 @@ static int validate_config(bip_t *bip)
 		}
 	}
 
+	if (strstr(conf_log_format, "\%u") == NULL)
+		mylog(LOG_WARN, "log_format doesn't contain \%u, all users'"
+			" logs will be mixed !");
 	return r;
 }
 
diff --git a/src/util.c b/src/util.c
index 32b5c49..74f35bf 100644
--- a/src/util.c
+++ b/src/util.c
@@ -42,7 +42,7 @@ int is_valid_nick(char *str)
 	while (*tmp != '\0' && (isalnum(*tmp) || *tmp == '-' || *tmp == '[' ||
 			*tmp == ']' || *tmp == '\\' || *tmp == '`' ||
 			*tmp == '^' || *tmp == '{' || *tmp == '}' ||
-			*tmp == '|'))
+			*tmp == '|' || *tmp == '_' ))
 		tmp++;
 	return (*tmp == '\0');
 }