Bug or misunderstanding: virtual_alias_maps + regexp:table + multiple dots in local-part

[cac2s]

*# cat /etc/debian_version *
8.4
*
# postconf mail_version
*mail_version = 2.11.3
*# postconf receive_override_options*
receive_override_options =

*# postconf virtual_alias_maps *
virtual_alias_maps = regexp:/etc/postfix/virtual/sub_mailboxes.regexp.cf 
proxy:ldap:/etc/postfix/virtual/traps.ldap.cf

*# cat /etc/postfix/virtual/sub_mailboxes.regexp.cf*
/^(.+)\.{5}.+@(example\.net|example\.com)$/     $1@$2

*# postmap -q postmaster.....t...@example.net 
regexp:/etc/postfix/virtual/sub_mailboxes.regexp.cf *
postmas...@example.net

*# postmap -q postmas...@example.net 
ldap:/etc/postfix/virtual/traps.ldap.cf *
u...@example.net

Based on this, if you send a letter to the 
postmaster.....t...@example.net, it should be delivered to the 
u...@example.net. Right?.. Postfix don't think so... a letter will be 
delivered to the postmas...@example.net

However, if you change regexp like this (replace "\.{5}" with "_{5}"):
//^(.+)_{5}.+@(example\.net|example\.com)$/     $1@$2/
then a letter will be delivered to the u...@example.net 
(postmaster.....t...@example.net -> postmas...@example.net-> 
u...@example.net)

In the rfc2822 (3.4.1. Addr-spec specification) there is nothing about 
limiting the number of dot-symbols in the middle of the local-part...

Who/what is wrong?

Thanks