$ischannel(<channel name>)

• This function returns 1 if <channel name> is a valid channel name, and 0 if it is not a valid channel name.
• A channel is “valid” if it begins with the plus character, ('+'), the hash character ('#'), the ampersand ('&'), or the bang ('!').
• This function does NOT tell you whether or not a channel exists.

Some scripts like to prefix the # character to a channel name if you try to join a channel that isn't valid, as a shortcut. Such as:

alias join (channel, ...) {
  if (ischannel($channel)) {
    //join $channel $*
  } else {
    //join #$channel $*
  }
}

1 if the argument is a valid name for a channel
0 if not.

$ischannel(#blah)              returns true
$ischannel(&blah)              returns true
$ischannel(blah)               returns false

This function first appeared in ircII-2.2pre1