Unknown Language Round 1 F. Domain

• Time : $O(|s|)$
• Space : $O(|s|)$

set channel {stdin}
 
gets $channel s
 
set len [string length $s]
 
set i 0
while {$i < $len} {
    set c [string index $s $i]
    if {! [string match {[0-9a-z.]} $c] } {
        puts "NO"
        exit
    }
    set i [expr $i + 1]
}
 
set i 0
while {$i + 1 < $len} {
    set c1 [string index $s $i]
    set c2 [string index $s [expr $i + 1]]
    if {[string compare $c1 .] == 0 && [string compare $c2 .] == 0} {
        puts "NO"
        exit
    }
    set i [expr $i + 1]
}
 
set c [string index $s 0]
if {[string compare $c .] == 0} {
    puts "NO"
    exit
}
 
set c [string index $s [expr $len - 1]]
if {[string compare $c .] == 0} {
    puts "NO"
    exit
}
 
set i [expr $len - 1]
while {$i >= 0} {
    set c [string index $s $i]
    if {[string compare $c .] == 0} {
        break
    }
    set i [expr $i - 1]
}
 
if {$i < 0} {
    if {$len == 2 || $len == 3} {
        puts "YES"
    } else {
        puts "NO"
    }
} else {
    if {$i == [expr $len - 3] || $i == [expr $len - 4]} {
        puts "YES"
    } else {
        puts "NO"
    }
}