Regex:
(?:(?:threads/[^.]*\.|showthread\.php\?t=)(?<thread>[0-9]*)(?:[^\n-]*-(?<page>[0-9]*))?|posts/|showthread\.php\?p=)(?:(?:#post-)?(?<post>[0-9]*))?

Test text:
[OTHER TEXT DON'T TOUCH]http://forums.spacebattles.com/threads/blahblah.1234/page-56#post-789[OTHER TEXT DON'T TOUCH]
FILLER TEXT

[OTHER TEXT DON'T TOUCH]http://forums.spacebattles.com/threads/blahblah.1234/page-56[OTHER TEXT DON'T TOUCH]
FILLER TEXT

[OTHER TEXT DON'T TOUCH]http://forums.spacebattles.com/threads/blahblah.1234[OTHER TEXT DON'T TOUCH]
FILLER TEXT

[OTHER TEXT DON'T TOUCH]http://forums.spacebattles.com/threads/.1234/page-56#post-789[OTHER TEXT DON'T TOUCH]
FILLER TEXT

[OTHER TEXT DON'T TOUCH]http://forums.spacebattles.com/threads/.1234/page-56[OTHER TEXT DON'T TOUCH]
FILLER TEXT

[OTHER TEXT DON'T TOUCH]http://forums.spacebattles.com/threads/.1234[OTHER TEXT DON'T TOUCH]
FILLER TEXT

[OTHER TEXT DON'T TOUCH]http://forums.spacebattles.com/posts/789[OTHER TEXT DON'T TOUCH]
FILLER TEXT

[OTHER TEXT DON'T TOUCH]http://forums.spacebattles.com/showthread.php?t=1234[OTHER TEXT DON'T TOUCH]
FILLER TEXT

[OTHER TEXT DON'T TOUCH]http://forums.spacebattles.com/showthread.php?p=789[OTHER TEXT DON'T TOUCH]
FILLER TEXT

Regex breakdown:
(?:                              # Noncapturing group containing the thread and post. Nonoptional
    (?:                          # Optional group containing the thread identifiers
        threads/[^.]*\.          #    threads/name-of-thread.
        |                        #    or
        showthread\.php\?t=      #    showthread.php?t=
    )                            # 
    (?<thread>[0-9]*)            # Named group containing the thread ID
    (?:                          # Optional group containing the page number
        [^\n-]*-                 #    /page-
        (?<page>[0-9]*)          #    Named group containing the page number
    )?                           # 
    |                            # or
    posts/                       # posts/
    |                            # or
    showthread\.php\?p=          # showthread.php.php?p=
)                                # 
(?:                              # Optional group containing the post ID
    (?:#post-)?                  # #post-
    (?<post>[0-9]*)              # Named group containing the post ID
)?                               #