back.php

<?php

// This assumes $text is the input we are parsing.
// I am using normal `echo`s here to output text, you will use whatever function you need.
if (preg_match('/^\/ba?ck(?:\s+(.*?))?\s*$/', $text, $result)) {
  // $text matched!
  if (isset($result[1]) && strlen($result[1]) > 0) {
    // $text included an optional back message to display.
    echo '<p><strong>Back:</strong> ' . $result[1] .'</p>';
  } else {
    // $text did not include any message, just show the default.
    echo '<p><strong>Back.</strong></p>';
  }
}

?>

info.markdown

What we look for:

/bck

/back

/bck Got my coffee.

/back Done in the bathroom.

Anti-whitespace:

The command (/bck or /back) can be followed by any amount of whitespace, this will happily be ignored. Even when a message is supplied all whitespace between the command and the message is stripped. This means all of the following commands are the same:

/bck     Baby stopped crying.

/back               Baby stopped crying.                      

Whitespace before the actual command is not allowed. This will not parse:

     /bck