Characters are supported but there is no class for it (like \pL for letters).

Re-opened by request of submitter.

Sorry it has taken me so long to get back to this, but at the 
time of reporting it I wasn't confident enough with Regular 
Expressions to dispute the bogus status. I have now asked for it 
to be re-opened as I'm confident it isn't bogus. Whilst 
InMusicalSymbols isn't supported, "Greek" is as are many other 
script types. The following code demonstrates this (I omitted the 
output in an attempt to avoid TL:DR).

<?php
header("Content-Type: text/html; charset=UTF-8");
$chars = array('Φ', 'a', '1');

foreach($chars as $char) {
  if( preg_match( '#\p{Greek}#u', $char ) ) {
    echo "{$char} is from the Greek script.<br/>";
  } else {
    echo "{$char} is NOT from the Greek script.<br/>";
  }
}

foreach($chars as $char) {
  if( preg_match( '#\p{invalid}#u', $char ) ) {
    echo "{$char} is from the Greek script.<br/>";
  } else {
    echo "{$char} is NOT from the Greek script.<br/>";
  }
}
?>

A full list of supported scripts can be found @ 
http://www.pcre.org/pcre.txt. If needs be I am happy to create a patch 
for the documents, but thought it only fair to re-open this bug in case 
further discussion is required first.

Patch? :)

Err. I switched it to "No Feedback" too soon... Has the attached patch been 
committed? It seems like a reasonable contribution.

No feedback was provided. The bug is being suspended because
we assume that you are no longer experiencing the problem.
If this is not the case and you are able to provide the
information that was requested earlier, please do so and
change the status of the bug back to "Open". Thank you.